Probing of Local Multifield Coupling Phenomena of Advanced ...csqs.xjtu.edu.cn/17.pdf · Probing of Local Multifield Coupling Phenomena of Advanced Materials by Scanning Probe Microscopy

REVIEW

1803064 (1 of 23) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advmat.de

Probing of Local Multifield Coupling Phenomena of Advanced Materials by Scanning Probe Microscopy Techniques

Tao Li and Kaiyang Zeng*

Dr. T. Li, Prof. K. ZengDepartment of Mechanical EngineeringNational University of Singapore9 Engineering Drive 1, Singapore 117576, SingaporeE-mail: [email protected]. T. LiCenter for Spintronics and Quantum SystemState Key Laboratory for Mechanical Behavior of MaterialsSchool of Materials Science and EngineeringXi’an Jiaotong UniversityShaanxi 710049, Xi’an, China

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201803064.

DOI: 10.1002/adma.201803064

1. Introduction

Functional materials are intrinsically responsive to external stimuli. A classic example is the piezoelectric material, in which electric potential is developed when the material is subjected to external mechanical stress. Functional materials may also show synergetic responses to more than one types of external stimuli,[1–3] which can be electrical, mechanical, magnetic, and/or chemical fields, as well as environmental effects from light,

The characterization of the local multifield coupling phenomenon (MCP) in various functional/structural materials by using scanning probe microscopy (SPM)-based techniques is comprehensively reviewed. Understanding MCP has great scientific and engineering significance in materials science and engineering, as in many practical applications, materials and devices are operated under a combination of multiple physical fields, such as electric, magnetic, optical, chemical and force fields, and working environments, such as different atmospheres, large temperature fluctuations, humidity, or acidic space. The materials’ responses to the synergetic effects of the multifield (physical and environmental) determine the functionalities, performance, lifetime of the materials, and even the devices’ manufacturing. SPM techniques are effective and powerful tools to characterize the local effects of MCP. Here, an introduction of the local MCP, the descriptions of several important SPM techniques, especially the electrical, mechanical, chemical, and optical related techniques, and the applications of SPM techniques to investigate the local phenomena and mechanisms in oxide materials, energy materials, biomaterials, and supramolecular materials are covered. Finally, an outlook of the MCP and SPM techniques in materials research is discussed.

Multifield Coupling

temperature, moisture, and atmosphere. The studies of multifield coupling phe-nomena (MCP) are important for func-tional and structural materials because they are often operated under several external stimuli simultaneously in real applications. In the area of multifield coupling, a group of researchers have dedi-cated to the computational and modeling works in order to explain the fundamental coupling mechanisms,[4–6] and the other stream of research has been focused on the experimental studies of MCP from macroscale to nanoscale.[1,7–10] MCP can be found in many different applications. For example, the electrodes in Li-ion battery (LIB) may undergo large mechanical stress/strain and heat generation during the electrochemical charging and dis-charging processes, which are the major failure mechanisms and safety concerns in battery operations.[11] Another example is the widely used piezoelectric or ferroelec-tric sensors and actuators, which are based on the phenomenon of electrically induced deformation or mechanical stress-induced

voltage.[12,13] At the nanoscale, complicated local phenomena may occur even when the material is under single stim-ulus.[14–18] In such case, piezoelectric responses may be com-plicated by charge injection from the nanoscale electrodes;[19] and the fracture behavior of the ferroelectric materials may be affected by electric field or domain switching processes.[20–22] Hence, understanding the MCP of functional materials at var-ious length scales, especially the localized effects, has important implications for both scientific and engineering disciplines.

The earliest MCP studies were probably on the multiferroics dated back to the 1950s, and the term multiferroic was coined in 1993.[23,24] Spaldin and Fiebig proposed a well-recognized illustration of MCP among the electrical (E), magnetic (H), and mechanical (σ) fields by forming a triangular relationship for multiferroics (Figure 1i).[25] The triangular relationship impli-cates the phenomena of piezoelectricity, flexoelectricity, spin-tronics, and so on. Later, Kalinin et al. extended the relationship into a pyramid by including the effects of chemical potential (µ) (Figure 1ii).[16] In fact, the coupling effects can be further extended to include other factors, such as light (hv) (optoelec-tronics), temperature (pyroelectricity and thermoelectricity), and gaseous atmosphere (such as physisorption and chem-isorption) (Figure 1iii). The direct coupling effects between different pairs of physical entities are summarized in Table 1.

Adv. Mater. 2018, 30, 1803064

http://crossmark.crossref.org/dialog/?doi=10.1002%2Fadma.201803064&domain=pdf&date_stamp=2018-10-10

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1803064 (2 of 23)

www.advmat.dewww.advancedsciencenews.com

Various types of techniques have been developed for MCP characterizations at different length scales. Classical methods to acquire piezoelectric coefficient, polarization value, or ferroelectric polarization hysteresis loop are based on the bulk material behav-iors under an external electric field. Nanoindentation is exten-sively used to measure the elasticity and hardness of materials, including the properties of the battery electrodes during charging and discharging processes.[26–30] However, nanoindentation can only obtain average results over a large area, but not the properties with spatial variation, such as the different responses from the grains and grain boundaries.[31] For high-resolution nanoscale characterization, the advantages of scanning probe microscopy (SPM) techniques start to evolve, which are discussed in detail later. At nanoscale, complex phenomena can arise even under single external stimulus. For instance, at a metal–semicon-ductor interface, charge injection due to highly localized electric field might cause the shift of Fermi level, hence changing the electrical conductance of the materials.[15,16,18,32] Another example is ZnO, the external electric field might cause the reorientation of the dielectric dipoles, which affected the resistive switching behavior, and further initiated a polarization switching phenom-enon similar to that of the ferroelectric materials.[33–40]

To study the localized MCP, the characterization techniques should have high spatial resolution, be able to apply multiple stimuli simultaneously at the same location of materials and to in situ measure the materials’ responses under such multistimuli, either individually or concomitantly. SPM techniques use a tinny sharp tip to characterize material properties with extremely high spatial resolution. The versatility of SPM techniques is also able to measure the different physical properties at the same location. Thus, they are the ideal tools to study the MCP of various mate-rials from nanometer to micrometer scales.[41–54] Since the inven-tion of the scanning tunneling microscope (STM) and atomic force microscopy (AFM), the two earliest functional modes in SPM family, a variety of techniques have been developed and widely used for characterizing different physical properties of

Tao Li obtained her Ph.D. in mechanical engineering from the National University of Singapore and spent three years as a postdoctoral research associate at the University of Nebraska-Lincoln, USA. She is a professional experimentalist of scanning probe microscopy techniques in the fields of calcified biolog-ical tissues, ferroelectrics and

their heterostructures with 2D materials, hybrid perovskite solar cells, and lithium-ion batteries. She has recently joined the School of Material Science and Engineering, Xi’an Jiaotong University, China, as a faculty member. Her current research focuses on multifield coupling applied in 2D/ferroelectric heterostructures and multiferroic nonvolatile memory devices.

Kaiyang Zeng is an associate professor at the Department of Mechanical Engineering, National University of Singapore (NUS). He obtained his Ph.D. degree in materials science from the Royal Institute of Technology (KTH), Sweden. His main research areas include using scanning-probe-microscopy-based techniques to study multifield coupling

phenomena in advanced materials, such as high-performance piezo/ferroelectric materials, biomaterials, supramolecular materials, oxide materials, and energy-storage materials.

Adv. Mater. 2018, 30, 1803064

Figure 1. Schematic diagrams demonstrating multifield coupling: i) electrical (E)–magnetic (H)–mechanical (σ) coupling; ii) electrical (E)–magnetic (H)–mechanical (σ)–chemical (µ) coupling; and iii) electrical (E)–magnetic (H)–mechanical (σ)–chemical (µ)–light (hv)–environment (Env: T, H2O, gas, etc.) coupling.



materials. Figure 2 shows the basic setups of some representa-tive SPM techniques. The most used SPM techniques include contact/noncontact/tapping-mode AFM,[41–44] conductive AFM (C-AFM) (Figure 2i),[45,46] Kelvin probe force microscopy (KPFM) (Figure 2ii),[47,48] electrostatic force microscopy (EFM),[49,50] magnetic force microscopy (MFM),[51,52] scanning thermal microscopy (SThM),[53–55] electrochemical AFM (EC-AFM),[56–58] piezoresponse force microscopy (PFM) (Figure 2iii),[59–64] electro-chemical strain microscopy (ESM),[64–73] contact resonance force microscopy (CR-FM) (Figure 2iv),[74–77] and multifrequency AFM such as AM–FM (amplitude modulation–frequency modula-tion)[78–81] and bimodal AFM,[82–85] and many others. Some of the SPM measurements results are illustrated in Figure 3. However, the major limitation of the SPM techniques is lack of structural or compositional information. A few recent studies have used ESM and SThM to indirectly map the different compositional phases of materials.[54,86] SPM has also been integrated with

Raman or IR spectroscopy in order to combine the advantages of high spatial resolution of SPM and the ability to quantify chem-ical information by Raman or IR techniques.[87,88]

We therefore, here, provide a comprehensive review of recent findings and developments of characterizing MCP using various SPM techniques. The primary focus is on the direct and complex couplings between electric field and other properties. At the end of the review, we also provide perspectives on the potential future developments of MCP and SPM techniques.

2. Multifield Coupling Phenomena of Advanced Materials

MCPs include different combinations of direct effect between two entities as listed in Table 1, such as electrome-chanical coupling (EMC), electrochemical coupling (ECC),

Adv. Mater. 2018, 30, 1803064

Table 1. Direct couplings between different pairs of physical entities. The focus here, electric couplings with other properties, is highlighted.

Stimulus Response

Stress/strain Electric potential Heat Light Magnetic field/spin Chemical reaction

Stress/strain/motion Piezoelectric/flexoelectric/

triboelectric effect

Mechanocaloric effect Mechanoluminescence Magnetoelastic

coupling

Mechanochemical

coupling

Electric potential/

current

Inverse-piezoelectric

effect/electrostriction

Joule heating/

thermoelectric

Electroluminescence Magnetoelectric

coupling

Electrochemical effect

Heat Thermal expansion/

stress

Thermoelectric/

pyroelectric

Thermoluminescence Thermomagnetic

effect

Thermochemical effect

Light Photostriction Photovoltaic/

optoelectronic

Photothermal effect Photomagnetic

effect

Photochemical effect

Magnetic field/spin Magnetostriction Magnetoelectric

coupling

Thermomagnetic effect Magneto-optic effect –

Chemical reaction Mechanochemical

coupling

Electrochemical effect Thermochemical effect Chemiluminescence –

Figure 2. Schematic diagrams showing the setups of some commonly used SPM techniques: i) C-AFM, ii) KPFM, iii) PFM, iv) CR-FM, and v) pc-AFM.



mechanochemical coupling (MCC), photoelectric cou-pling (PEC), thermoelectric coupling (TEC), and magneto-electric coupling (MEC), also include the multientity coupling such as electro-mechanochemical coupling (EMCC) and photo-electromechanical coupling (PEMC). There are also indirect MCPs, such as electric field or optical illumination caused heating effect that leads to changes in mechanical strain, magnetic spin, or electric polarization. A variety of devices have been fabricated based on MCP. Typical examples are the piezoelectric sensor, actuator, and energy harvester, which are based on the EMC, i.e., voltage developed by applied stress or mechanical strain induced by electrical field. Fer-roelectric material is a special type of piezoelectric material, which exhibits spontaneous polarization that can be switched between at least two stable states by external electric field. External stress can also reverse the polarization direction.[89] The feature of polarization reversal enables ferroelectric thin films to be used as nonvolatile digital memories, in the form of capacitor or tunnel junction.[90,91] Frictional force, another type of mechanical stimuli, can also induce electric potential based on triboelectric effect, which has been used for energy harvesting.[92,93] Another example of MCP is rechargeable bat-tery, which is based on the ECC principle, where chemical energy is transformed to electrical energy. Thermoelectric materials or devices generate electric potential by temperature changes with the TEC mechanisms.[94,95] Photovoltaic solar cells convert photoenergy into electrical energy based on PEC principle.[96,97] Recent researches using 2D materials in light

emitting diode (LED), photodetector, and phototransistor well illustrated the applications of PEC effects.[98–100] Spin–orbit coupling is a manifest of MEC, which has been intensively investigated in magnetic tunnel junction,[101,102] topological insulator,[103] superconductor,[104] and many other materials.

Besides the coupling between two fields, multientity coupling also exists in many functional materials. In such case, photo-sensitive ferroelectric crystals may be used for multisource energy harvesting simultaneously from thermal, mechanical, and photo-energy.[105] Multiferroic materials intrinsically exhibit both ferromagnetic and ferroelectric orders, which offer the advantages of controlling the magnetic properties by electric field instead of magnetic field.[23] These coupling behaviors may also influence each other, i.e., one coupling phenomenon may cause another coupling effect. For instance, when ZnO was switched to high resistance state by a large electric field, its polarization dipoles can be reversed by a smaller electric field in a similar way as the polarization switching characteristics in the traditional ferroelectric materials.[37–40]

2.1. Electromechanical Coupling

EMC refers to the mutual conversion between electrical and mechanical energy via a variety of mechanisms, such as piezo-electric, ferroelectric, electrostriction, and triboelectric. EMC has been observed in many organic and inorganic materials[106–109] and is believed to be a universal phenomenon in almost all

Adv. Mater. 2018, 30, 1803064

Figure 3. i) Schematics of the principle of intensity-modulated KPFM: A) light-modulating periods of the LED, with the red trace being the modulation with a low frequency and blue trace being the modulation with a high frequency; B) oscillations of the sample surface potential as a result of the LED modulation. The horizontal lines are the time-averaged values recorded by the instrument. i) Reproduced with permission.[278] Copyright 2014, American Chemical Society. ii) Schematic illustration of waveforms of: a) light intensity and b) corresponding photovoltage as a function of time t. ii) Reproduced with permission.[279] Copyright 2008, AIP Publishing. iii) Schematic illustration of PFS waveform (a), PFS measured amplitude and phase loop for PZN-PT single crystal sample (b), and calculated PR loops for PZN-PT single crystal (c). iv) Schematic diagram showing the typical approaching and retracting force–distance curves by AFM measurements.



biological systems.[110] The earliest studies of piezoelectric prop-erties of biological systems were on the wood and bones by Fukada.[111,112] In 2000, he summarized the major progress of piezoelectricity of polymer and biopolymer works conducted in the past 50 years.[113] The first PFM study on a biological system was on bone samples in 2004;[114] since then the studies of EMC of biological systems at the nanoscale have been emerging quickly, such as in teeth, protein, collagen, butterfly wings, sea-shells, cartilage, and the aortic wall.[110,115–117] At the nanoscale or molecular level, the structural symmetry can be changed or broken at the surface and/or interface, which may introduce piezoelectric behavior even in nonpolar materials. The EMC effect will therefore affect the functionality of the materials. In biological systems, EMC may also control the mechanisms of tissue growth at its fundamental level due to the polar bonds in complex organic molecules. Examples are piezoelectricity in calcified and connective tissues, and more complex EMC involved the voltage-controlled muscular contractions, cell electromotility, electrimotor protein, and more.[118] Among the SPM techniques, PFM is particularly designed to study the piezoelectric and ferroelectric properties for various materials and devices. Its application has now been expanded to study the general EMC phenomena in nonpiezoelectric materials.

2.2. Electrochemical Coupling

ECC generally refers to the interaction or interconversion between chemical and electrical energy. ECC is widely involved in ionic channels of biological tissues, gas sensors, material corrosion, and energy storage systems. In energy storage systems, ions migration or diffusion conditions and electro-chemical reactions control the fundamental properties and performance of electrochemical devices, such as fuel cells and batteries. Bias-induced phase transition is another ECC-related phenomenon,[119] such as electrochemical memory effect, which involves bias-induced phase transition, charge and mass motion, electrochemical reaction, thermal exchange, and interface effects. Another important issue is that when conducting SPM measurements on ferroelectric thin films, electric bias may induce a series of electrochemical reactivities on the film surface, which may cause reversable ionic surface charging or irreversible surface degradation.[120] This type of electrochemically active ferroelectrics manifests the multifield EMCC phenomena. Generally speaking, EC-AFM has been the primary SPM technique to study ECC for many years.[56–58]

2.3. Photoelectric Coupling and Photo-Electromechanical Coupling

When a photon strikes an electron, depending on the photon energy, the electron can either absorb the energy, be exited to a higher-energy orbital slot, or be knocked off from the parent atom leaving an empty orbital slot. If another electron jumps into the vacant orbital slot, photon with specific frequency can be emitted, i.e., radioactive decay. Alternatively, exited elec-trons and holes can be recombined without light emission (nonradioactive decay) if there is no efficient charge separating

mechanism. Photovoltaic and optoelectronic devices, such as solar cells, photodetectors, phototransistors, and LEDs, are built based on such fundamental interactions between photons and electrons.

Solar cells utilize the photovoltaic effect to generate photoin-duced voltage to power devices. One of the critical parameters is power conversion efficiency (PCE). Currently, the highest PCE was about 26% obtained from a single silicon heterojunction.[121] For inorganic–organic hybrid perovskite solar cells, the certified highest PCE was 22.1% in small cells.[122] Understanding the fundamental material behaviors and mechanisms is therefore necessary to further increase PCE and to invent innovative and reliable devices. The nanoscale inhomogeneity of the materials, such as grain boundaries, interfaces, defects, and nonuniform doping, critically affects the macroscopic device performance. SPM is an ideal tool for, but not limited to, solar cell charac-terizations by providing various correlated properties mappings and spectroscopies with high spatial resolution.

SPM techniques for solar cell researches typically include KPFM (Figure 2ii), photoconductive-AFM (pc-AFM) (Figure 2v), and EFM. EFM and KPFM are commonly used to image spa-tial variation of contact potential difference (CPD), which origi-nates from the electrostatic interaction force between the SPM tip and sample surface. CPD reflects electronic and optoelec-tronic properties such as work function, charge density, doping type, charge-carrier concentration, surface dipole, and elec-tron affinity.[123] pc-AFM is analogous to C-AFM but operated in optical illumination and is used to measure photocurrent, open-circuit voltage, I–V spectroscopy, and to extract maximum power and fill factor.[124–127] Thus, the relationships among morphology and electrical and photoelectric properties can be established using these in situ techniques. Furthermore, light can be directed to the sample surface in various ways, and the measurements can be done in different atmospheres to prevent sample degradation or to eliminate effects from oxygen and moisture. In addition, one can also study the material perfor-mance as a function of temperature or external electrical field, which is important for optoelectronic materials and devices. There are numerous works using SPM techniques to inves-tigate the spatial variation of functionality of solar cells and optoelectronic devices, such as to correlate structural and elec-tric properties of organic solar cell materials,[128] to identify the grain boundary barrier for hole transport in solar cell-quality CdTe,[129] and to study the phase separation,[130] ion migra-tion,[131] and facet dependent efficiency variation.[127] Similarly, these SPM techniques can also be applied to characterize pho-todetector, phototransistor, and photodiode.

Electrons can be optically excited in all semiconductors (including ferroelectrics) by sufficiently high photon energy, which enables the multifield PEMC. The photovoltaic behav-iors of ferroelectrics are generally originated from the bulk and anomalous photovoltaic effects with two distinctive features.[132] One is that the polar domains in ferroelectrics intrinsically facilitate charge separation of the excitons, and the other is that the generated photovoltage can be higher than the bandgap of the ferroelectric materials probably due to the presence of the domain walls.[132] The large bandgap of ferroelectrics hinders their solar cell applications, but recent works demon-strated the bandgap engineering via tuning the composition of

Adv. Mater. 2018, 30, 1803064



ferroelectrics, which has achieved a proper bandgap for solar cell application.[133,134] To fully understand the material behav-iors, the coupling mechanisms between the optically excited carriers and the ferroelectric domains and domain walls need to be investigated at the nanoscale. Hence, in addition to the pre-viously mentioned techniques for photovoltaic measurements, PFM can also be used for ferroelectric domain observation and manipulation.

3. Scanning Probe Microscopy Techniques

Since the invention of AFM by Binnig et al. in 1986,[135] many functional modes have been developed and hence formed a big family named scanning probe microscopy. STM, KPFM, EFM, C-AFM, and scanning microwave impedance (sMIM) are commonly used to characterize electrical properties of mate-rials. Force–distance curve measurements, CR-FM, bimodal AFM and AM–FM modes are widely used to characterize the mechanical properties of materials, such as elastic modulus and loss modulus. ESM and EC-AFM are used for electrochem-ical-related property characterizations. PFM and associated spectroscopy techniques (PFS or SS-PFM) are widely used to study piezoelectric and ferroelectric materials (Figure 3iii). Thermal properties, such as local thermal expansion or conduc-tivity, can be probed using SThM. Optically induced materials behaviors, such as photocurrent- and wavelength-dependent phenomena, can be studied by pc-AFM or combining SPM with optical microscopy. In addition, various customized setups are developed, such as integrated SPM-Raman and SPM-IR. SPM with special excitation techniques are also invented, such as interferometric displacement sensor (IDS),[136] blueDrive photo-thermal excitation,[137] dual AC resonance tracking (DART),[138] band excitation,[139–142] PeakForce,[143] and others. Furthermore, SPM can be placed inside a glovebox or integrated with an ultrahigh vacuum (UHV) system in order to precisely control the experimental environment or to conduct measurement at cryogenic temperature.

To study MCP of materials at nanoscale to microscale levels, one should apply multiple SPM modes simultaneously or individually at the same location of the sample. Taking multiferroics as an example, C-AFM can be used to study the resistive switching, followed by KPFM to measure the surface charges or screening charges due to the applied electric field, and finally using PFM to study the domain switching. These measurements can easily be performed in situ at the same location using SPM so that multiple properties can be corre-lated seamlessly with each other. Another example is to apply multifrequency technique to measure the changes of mechan-ical properties caused by the application of electrical field or polarization switching. Such structural–mechanical–electrical correlations are critical for many applications at device level.

3.1. SPM Techniques for Electrical Properties Measurements

Measurements of electrical properties were among the earliest developed SPM techniques. STM is the historically first devel-oped SPM technique to reveal the true atomic structures, which

uses tip–sample tunneling current as the feedback mechanism, but it is only applicable to conductors. Thus, AFM is developed to acquire the atomic structure of semiconductor and insula-tors. Later on, C-AFM is developed based on the contact mode AFM to simultaneously measure topography and current by maintaining a constant tip–sample interaction force using a feedback loop. During C-AFM measurements, a voltage is applied between the sample and tip, and the height profile and current flow are recorded at each pixel to form the topography and current images. Local current–voltage (I–V) curve can also be obtained by applying a voltage swept (Figure 4i) either at a single point or form an image using characteristic parameters of the I–V curve (i.e., I–V curve at each pixel of an image). To obtain comprehensive electrical information of the sample sur-face, both out-of-plane and in-plane current can be measured by varying the experimental setup. The concomitantly obtained cur-rent and morphology image can provide more detailed informa-tion of phase separation, charge generation, and transport.[125] C-AFM can be applied to both conductors and semiconductors, hence it is widely used over STM.

KPFM is another SPM technique to measure electric proper-ties such as work function, surface charge, and doping profile. It directly measures CPD between the metal-coated tip and sample surface, which equals the voltage to nullify the first resonant component of the tip–sample electrostatic force.[47] CPD is defined in Equation (1) and reflects the work function difference between the conductive tip and sample

/ /CPD s tV e e( )= ∆Φ = Φ − Φ (1)

where e is the elementary charge, and Φs and Φt are the work function of the sample and tip, respectively. Under UHV condi-tions, KPFM measurement can be used to determine the abso-lute work function value of the sample if the work function of the tip is known. Even under ambient condition, for certain materials, KPFM can still be used to determine the relative work function values.[144] In addition to work function, KPFM also reveals information of doping, charge dynamics, band bending, and more electronic properties, which are discussed in detail in Section 3.4.

EFM is another powerful technique for electrical proper-ties characterization. It measures the variations in electrostatic force and/or force gradient that arises from local differences in chemical potential and/or dipole moment.[145] EFM has been used to quantitatively map charge trapping within a semicon-ductor layer or vacancy movement in oxides.[146] Since it can be used to characterize the defects in the materials, it is extremely useful to study the degradation mechanisms in many photovol-taic materials, especially solar cell materials.

There are also some relatively newly developed techniques, including scanning microwave impedance microscopy and scanning impedance microscopy (SIM).[147,148] SMIM is primarily used in semiconductor industry to quantify permit-tivity, capacitance, resistance, and dopant concentration and locate the imperfections or electrical inhomogeneity.[149–151] SIM measures the local AC transport behavior. It was used to study the local impedance variation in different ionic solids, semiconductor devices, polymer composites, and biomaterials such as proteins.[152,153]

Adv. Mater. 2018, 30, 1803064



3.2. SPM Techniques for Mechanical Properties Characterization

AFM relies on the measurement of surface forces acting on a sharp tip in close proximity to a surface. These surface forces exist between any tip and sample surface in the range of pN to µN. By measuring the forces as a function of tip–sample separation (force–distance curve (Figure 3iv)), various informa-tion related to the mechanical properties can be obtained.[154] However, such method suffers from the uncertainties in the cantilever elasticity and the tip shape and dimensions, hence it is difficult to accurately quantify mechanical properties from the tip-based force–distance curve. CR-FM and recently devel-oped AM–FM techniques are intrinsically quantitative.[77] In CR-FM, sample is firmly attached to a resonator that can ini-tiate the ultrasonic oscillation of the sample. Such oscillation is probed by an AFM tip in contact with the sample surface in a normal static force. Image of contact resonance frequency is used to extract the elasticity of sample by taking account the cantilever dynamics and tip–sample contact mechanics. In order to quantify the elasticity of the tested sample, a reference sample with known elastic modulus in the similar range as that of the tested sample is required. CR-FM technique can obtain the high-resolution images of elasticity mapping, which is important information for highly inhomogeneous materials.[75]

AM–FM technique excites cantilever at two resonant fre-quencies simultaneously, where the lower eigenmode is ampli-tude modulated (AM) and the higher eigenmode is frequency

modulated (FM).[78–81] The AM mode yields the information as that from a typical tapping mode AFM, while the FM mode with an additional phase-lock loop delivers the information of con-tact resonant frequency and reflects energy loss at each pixel. Similar to CR-FM, the frequency signal is an exquisite probe of the elastic tip–sample interaction, with higher frequency value corresponding to higher contact stiffness or elastic modulus, and vice versa. A reference material with known elastic modulus is also required to quantify the elastic modulus of the material. Due to the intermittent contact operational mode, AM–FM has better resolution and sensitivity than CR-FM, with less sample preparation. It can be used to quantitatively evaluate elastic mod-ulus, loss modulus, and loss tangent. In authors’ group, CR-FM combined with BE technique (denoted as BE-CR-FM) was used to characterize elastic modulus of abalone shell,[75] and AM–FM technique was used to measure the elasticity of nanoparticles,[155] collagen in bones,[78,156] metal–organic framework (MOF) nanocrystals,[79] and electrode materials for Li-ion batteries.[157]

3.3. SPM Techniques for Chemical-Related Properties Characterization

Recent works have shown remarkable developments in obtaining chemical related information using SPM techniques. The traditional method is to combine electrochemical cell with AFM to form an EC-AFM but working in an independent way:

Adv. Mater. 2018, 30, 1803064

Figure 4. i) Average current–voltage (I–V) curves of NiO thin film measured by C-AFM in ambient air, synthetic air and argon gas. The inset shows the bias waveform applied for I–V measurements. ii) FORC-IV measurements on NiO thin film sample: a) applied bias waveform, b) average FORC-IV curves as a function of the bias, and c) FORC-IV loop area as a function of the bias amplitude. i,ii) Reproduced with permission.[161] Copyright 2018, American Chemical Society. iii) Local I–V curves measurements on: a) ZnO:Cu (2 at%) and b) ZnO:Cu (8 at%) thin films by using C-AFM, showing the resistive switching phenomena in ZnO:Cu samples. The inset shows the shift of the current values at “set” and “reset” processes of the I–V measurements for ZnO:Cu (8 at%) sample. iii) Reproduced with permission.[37] Copyright 2017, Elsevier.



electrochemical cell is controlled by an external potentiostat sta-tion, and AFM tip is used to profile the morphology changes induced by the electrochemical reaction of the materials.

ESM is a widely used SPM technique to acquire chemical related information.[64–73] It uses the intrinsic relationship between ionic concentration and molar volume to probe electric field induced ions movement and thus the local deformation of materials. ESM technique can be operated in liquid or air. In ambient air conditions (humidity ≥ 30%), a thin film of water molecules covers tip and sample and may form a meniscus at the tip–sample junction. In this case, the water molecules may act as electrolyte to promote the electrochemical reaction in active materials. ESM is operated in the similar way as PFM, both measures electric field induced surface deformation, but based on different mechanisms. To distinguish ECC-induced and EMC-induced responses, we introduced a closed cell during SPM measurements for atmosphere control, allowing continuous flow of synthetic air or argon (Ar) into and out from the cell. Synthetic air (21% oxygen and 79% nitrogen, with water <5 ppm) maintains a nearly water-free environment, and is ideal to clarify the effects of water on SPM measurements in ambient air. The ECC will be hindered or even disappear in the water-free environment, while PFM responses should be persist but with changed screening conditions. In the inert Ar envi-ronment, oxygen and other active gases are eliminated, which is ideal for the studies of intrinsic material behaviors without effects from water, oxygen, nitrogen, and other active sub-stances. We have adopted this environmental control method in our studies of oxide thin films, Li-ion battery materials, and ferroelectric materials.[35,37–40,157–162]

Scanning thermo-ionic microscopy (STIM) is another newly developed technique to probe local electrochemical phenomena with nanoscale resolution. Instead of electrical stimulation in ESM, STIM measured Vegard strain induced by thermal oscil-lation via a heated scanning probe.[163,164] One advantage of STIM is the usage of electrically insulating tip that prevents complication from the electromechanical effects. It also helps to distinguish linear strain, such as thermal expansion, from nonlinear ionic and electronic defects induced strains via adopting the fourth harmonic oscillation signal.

Another major effort toward relating high-resolution physical properties and quantified chemical information is to integrate AFM with either Raman spectroscopy or IR technique.[87,88] AFM-Raman or AFM-IR usually comprise two independent systems that are colocalized so that Raman or IR measurements can be performed at the same area scanned by AFM. The seam-less interfacing of AFM and Raman/IR allows the high spatial resolution correlation between SPM-mapped physical proper-ties with quantified chemical properties. In such systems, a monochromatic light excites sample, and the photon will be absorbed, reflected or transmitted. Raman measures the scat-tered photons, while IR measures the adsorption. AFM-IR has been applied in the studies of CH3NH3PbI3 hybrid perovskite solar cells to reveal its chemical heterogeneity[165] and ferroelas-ticity,[166] while AFM-Raman has been widely used in polymers characterizations.[167] The two newly developed techniques and related applications were recently reviewed by Fu and Zhang.[168]

Besides probing the electrochemical activities of materials, SPM is versatile enough to trigger local chemical reactions,

mainly via applying strong localized electric field (field-induced chemical reaction) or using the heated AFM tip (thermo-chemical nanolithography) or chemically modified tips. These measurements are usually assisted by water meniscus. These techniques have been applied in a variety of material systems, such as patterned oxidation of materials, deposition of semi-conductors, and thermal reduction of graphene oxide.[169–172]

3.4. SPM Techniques for Optical Properties Characterization

The nanoscale phenomena, including dynamic behavior of photovoltaic and optoelectronic devices can be investigated by using various SPM techniques, including pc-AFM, customized KPFM and EFM.

pc-AFM is a powerful tool to map photocurrent and I–V spectroscopy with nanoscale spatial resolution (Figure 2v). It is basically the C-AFM conducted at zero electric bias with con-trolled optical illumination. During the pc-AFM measurement, sample can be illuminated from top, side or bottom depending on how the illumination system is integrated with the SPM. If tunable monochromatic light source, filter and shutter are used in the system, effects of the wavelength and light intensity, and the photocurrent dynamics can be characterized. At zero bias, the detected current is the short-circuit current ISC from the photovoltaic effect only. By maintaining a zero ISC via sup-plying a DC bias, the open-circuit voltage VOC can be obtained. I–V curves can be measured at specific locations by supplying standard or arbitrary drive voltage and/or light pulses. Hence the maximum power, fill factor, local efficiency can be calcu-lated and mapped with the morphology, and the inhomogeneity inside the structure can be revealed.[124,127] Metal-coated probes or substrates of different work functions with small bias stimu-lation can be used for either hole or electron collection.[173] To quantify the current value, C-AFM/pc-AFM must be calibrated and applied carefully for each new material system.

KPFM usually relies on a feedback loop to electrically neu-tralize CPD. Besides mapping of equilibrium electronic phenomena, charge-carrier dynamics and transient phenomena are also important for understanding the material functionality and limitations, especially after bias or optical excitation. The response time of KPFM normally is on the orders of tens of milliseconds (the response time of the cantilever oscillation to external change can be written as τ = Q/(πf)), and this is usu-ally not adequate to observe fast dynamic processes. In recent years, improved KPFM techniques were developed to track the fast excitation and relaxation of electric pulse or photon induced charge-carrier and ionic dynamics in the time scale of 10–20 ns, which is much faster than the KPFM feedback elec-tronics.[123] The first report of time-resolved KPFM (trKPFM) was to measure the time-dependent surface photovoltage (CPDlight − CPDdark) of chalcopyrite solar cell.[174] The spatially resolved charge carrier dynamics and ion migration observed by trKFPM were usually in the time scale of seconds to min-utes.[174] Later, sixteen (16) ns for one image has been real-ized using heterodyne technique for fast imaging KPFM.[175] Furthermore, intensity-modulated (IM)-KPFM technique can visualize much more rapid photocarrier dynamics (Figure 3i,ii). It measures time-averaged CPD at different modulation

Adv. Mater. 2018, 30, 1803064



frequencies in response to modulated optical excitations. Both the decay and build-up processes of photocarriers in solar cell materials can be visualized.[176] If the recombination rate is slower than the modulation frequency, surface potential will be higher than those with sufficient decay time. Currently, pump probed (pp)-KPFM held the record of measurable decay time of about 1 ps.[177] The concept of pp-KPFM is to detect the CPD during the short selected times, with a sinusoidal envelop and then move this time slot with respect to a square wave of pump pulse excitation to the sample.[178] This technique was well demonstrated on a low-temperature grown GaAs, the carrier lifetimes of which was in the range of picosecond.[177]

EFM is another SPM technique that works based on the tip–sample capacitive gradient. Comparing to KPFM, EFM oper-ates in open loop and thus is intrinsically faster than KPFM. Ginger et al. demonstrated that time-resolved EFM (trEFM) could measure the charge dynamics with time resolution of about 100 µs in polymer solar cells without any external con-troller.[179] When light was turned on, the local surface potential and tip–sample capacitive gradient were altered, and eventually caused resonance frequency shift. By tracking the frequency shift, when light was turned on at each point, the photoinduced charging, recombination and trapping could be revealed. Charge generation and separation were typically fast processes that could not be probed by this type of trEFM, but it was suit-able to track release of trapped charges, which might take hours depending on the material system. trEFM has also been used to study photo-oxidation or degradation of organic solar cell mate-rials.[180] The fastest time resolution of trEFM was about 10 ns, with acquisition of the full data set of cantilever oscillation by digitizing the oscillation signal up to 50 MHz.[50] However, this technique requires long acquisition time and complicated post data processing.[181]

3.5. SPM Techniques for Electromechanical Characterization

PFM and its spectroscopy mode are widely used to study the EMC phenomena in piezo/pyro/ferroelectric materials. PFM is a contact mode technique, during which a sinusoidal voltage in the form of VDC + VAC cos(ωt) is applied to sample through a con-ductive tip (usually Pt-coated Si tip). According to inverse piezo-electricity or ionic electromigration, sample surface will vibrate at the same frequency and the sample oscillation is captured by detecting system and lock-in amplifiers through the same tip. The critical information delivered by PFM measurement are amplitude, phase angle, contact resonance frequency, and Q-factor. Amplitude is the electric field induced deformation/strain that is usually proportional to the piezoelectric constant. Phase angle reveals the relative polarization orientations of the ferroelectric domains. Contact resonance frequency reflects the tip–sample contact stiffness, and Q-factor represents the energy dissipation of the tip–sample interactions. PFM can be applied in both vertical (v-PFM) and lateral (l-PFM) direc-tions, which can be further formed into the vector-PFM or 3D PFM images. In the multidimensional PFM, each pixel is rep-resented by a vector with magnitude and orientation to form a vector matrix that delivers more information of domain patterns and dynamics. In addition, multifrequency techniques such as

DART[138] and BE[139–142] can be implemented with PFM to track the shift of contact resonance frequency that is caused by varia-tion of tip–sample stiffness during the PFM measurements.

PFM spectroscopy techniques (PFS or SS-PFM) can be used to obtain the information of domain switching characteristics in ferroelectric materials (Figure 3iii), either at individual point (PFS) or a mapping (SS-PFM). A typical phase change (φ) loop and butterfly-shaped amplitude (A) loop can be obtained by these techniques. The piezoresponse (PR) loop can be calcu-lated from the amplitude and phase data via the relationship of PR = A × cos(φ). From the PR loop, one can obtain numbers of parameters that describe the ferroelectric switching behavior, including coercive bias, imprint, remnant piezoresponse, work of switching, and internal field caused shifting. These parameters can be extracted from the PR loop at each pixel and form corresponding images.

In general, any types of deformation caused by electric field can be categorized as EMC, although the deformation may be originated from different mechanisms, and complicated by capacitive coupling, laser spot positioning on the cantilever and frequency dependent effects.[182,183] Groups of scientists have been dedicating to develop new techniques or theories to differentiate these mechanisms and to unambiguously quantify the electrically induced deformation for years. Li’s group has developed a method to acquire the first and second harmonic strain responses of PFM in order to distinguish piezoelectric-dominated (linear to applied electric field) and nonpiezoelectric (quadratic to applied electric field) mechanisms.[184,185] They have also interpreted the nonresonant phase signal of ESM between the ferroelectric and nonferroelectric materials to determine the sign of electrochemical strain.[65] Moreover, ferroelectric-like behaviors induced by charge injection and electrostatic force has been discriminated using contact Kelvin probe force microscopy (cKPFM) technique,[186] and a simple AC sweep till high ampli-tude also helped to differentiate piezoelectric and nonpiezoelec-tric responses.[187] More related information can be found in the review article by Seol et al.[188] On the other hand, a newly devel-oped SPM technique adopting a laser Doppler vibrometer (LDV) was able to accurately measure the cantilever dynamics and thus contributed to quantify electromechanical strain.[136]

4. Examples of Material Systems Investigated Using SPM Techniques

4.1. Oxide Materials

Oxides represent an important category of material used in many applications, such as digital memories, sensors and actuators, solar cells, and many more. With the miniaturization of the devices, nanoscale phenomena or quantum effects start to dominate, and SPM techniques are ideal to study these oxide thin films, heterostructures and devices.

4.1.1. Piezoelectric and Ferroelectric Materials

Ferroelectric crystals have been used for sensing and actu-ating for decades. Recently, ferroelectric thin films have been

Adv. Mater. 2018, 30, 1803064



employed in various nanoelectronic devices, such as ferroelec-tric tunnel junctions (FTJ) and field effect transistors (FET). The ferroelectric polarization field has been used to modulate conductivity of 2D materials or tuning magnetization.[189–192]

PFM is the primary tool to investigate the nanoscale fer-roelectric domains and polarization dynamics for various piezoelectric and ferroelectric materials such as PZT, relaxor-based piezoelectric materials (PZN-PT or PMN-PT), mul-tiferroic BiFeO3 (BFO) and organic ferroelectrics. We have applied PFM in many material systems, including doped/undoped BFO thin films, PZN-PT single crystal, CH3NH3PbI3 perovskite, MOF nanocrystals, poly(vinylidene fluoride) (PVDF) film, and ferroelectric-like copper-doped zinc oxide (ZnO:Cu) thin films.[32,35,37,108,193–199] Taking the PZN-PT single crystal as an example, the out-of-plan domain structure and the corresponding surface potential are illustrated in Figure 5i. By conducting v-PFM and l-PFM, we have obtained the 3D vector domain structure of PZN-PT. From the SS-PFM measurements on PZN-PT, imprint variations in oppositely oriented domains, and domain evolution processes due to heating or environ-mental conditions have been evaluated.[108] For general piezo-electric or ferroelectric materials, PFM can be used to observe the pristine domain states, in terms of polarization direction (phase image) and bias-induced deformation (amplitude image, ideally is proportional to piezoelectric coefficient). PFM only presents relative polarization directions with ≈180° phase angle difference in both out-of-plan and in-plan measurements. Elec-tric poling is usually required to find the definite polarization orientation. In addition to observing the domain states with

minimum required AC bias stimulation, ferroelectric domains can be switched by applying DC bias that is higher than the coercive field of the tested material. This can be achieved by imaging while applying high DC bias to a conductive tip in con-tact with sample, or via local SS-PFM measurement (DC voltage sweep, similar to the macroscopic ferroelectric hysteresis loop measurements). SS-PFM presents amplitude and phase loops (Figure 3iii) that demonstrate strain and polarization direction variation versus DC bias, respectively. The combined domain manipulation and observation using PFM is commonly used to evaluate polarization reversal dynamics, retention, and endur-ance. Other SPM techniques such as KPFM, C-AFM and EFM can also be used to complement the PFM results by providing charge and conductivity information.

4.1.2. Multiferroic Materials

A remarkable feature of multiferroics is their intrinsic magne-toelectric coupling. Magnetoelectric now refers to any type of coupling between magnetic and electric properties, which can be coupled independently or convoluted in a single-phase mul-tiferroic or via heterostructures. Multiferroic is the type of mate-rial exhibits both ferromagnetic and ferroelectric orders in the same material or phase. However, magnetic and electric orders are intrinsically opposing each other because the d electrons in transition metal are essential for magnetism but are not favorable for off-center ferroelectric distortion.[200] Thus, addi-tional driving force retaining multiferroicity is necessary. Four

Adv. Mater. 2018, 30, 1803064

Figure 5. PFM/ESM measurements on various materials. i) PFM images of the PZN-9%PT single crystal sample: a) surface topography of fine polished sample; b) PFM phase image; c) KPFM CPD image. All of the images were measured in the same region of pristine crystal. i) Reproduced with permis-sion.[108] Copyright 2016, Elsevier. ii) ESM images of NiO thin film after applying a 6 V DC bias on an 8 × 8 grid over 3 × 3 µm2 area: a–c) in argon gas and f–h) in synthetic air. ii) Reproduced with permission.[161] Copyright 2018, American Chemical Society. iii) Schematic drawing of the ESM-induced expansion (a) and contraction (b) on TiO2 thin film; c) DART-ESM topography image of pristine TiO2; d) resonance amplitude image and e) resonance Q-factor image after the application of a DC bias of −9 V on a 4 × 4 grid within the scanning area at ambient condition. iii) Reproduced with permission.[159] Copyright 2016, The Electrochemical Society.



identified mechanisms supporting the formation of multifer-roicity are lone-pair, geometric ferroelectricity, charge ordering, and spin-driven.[23] Among lone-pair systems, BFO is the only single-phase multiferroic material at room temperature. BFO thin film has large electric polarization (≈100 µC cm−2) and pro-nounced magnetoelectric coupling,[201] thus it has induced an upsurge of research on the bare BFO and its heterostructures in the past two decades.

One of the research focuses in this area is to control mag-netization using electric field. Ramesh and co-workers have reported a series of remarkable works on intrinsic ferroelectric and ferromagnetic coupling of single-phase BFO,[201,202] electric control of magnetization reversal in BFO–CoFe2O4 nanostruc-ture, BFO–CoFe dot, and BFO heterostructures at room tem-perature.[203–205] SPM techniques have played important roles in these works and are among those pushing the scaling limit to nanoscale. PFM and MFM are usually applied concomitantly to correlate ferroelectric and magnetic domains in pristine state, after electric bias or during magnetic field application. MFM operates in a noncontact mode to obtain magnetic domain of material surface via the magnetic force interaction between a magnetized tip and magnetic sample.[52] A new technique with better spatial resolution than MFM called piezomagnetic force microscopy has also been developed.[206] The intrinsic high resolution of SPM technique is ideal for domain wall studies. Domain wall conductivity of BFO thin film was examined by C-AFM,[207] and cryogenic MFM was used to probe the vortex domain walls of multiferroic h-ErMnO3.[208] The domain wall functionality in multiferroic has been nicely presented in a review article by Meier.[209] On the other hand, there are only limited number of room temperature single-phase multifer-roics and they usually exhibit weak magnetoelectric coupling. Thus, the heterostructures of ferroelectric and ferromagnetic layers with more options of structural combination are another focus of interfacial magnetoelectric coupling studies, such as Fe3O4/PZT,[210] FeGaB/NiTi/PMN-PT memory,[211] CoFe/Cu/CoFe/BFO spin valve,[212] Pt/CoFe/BFO,[213] and so on.

In recent years, skyrmions as potential information-holding bits have attracted much attention in the field of magnetic data storage since the dynamic creation at room temperature in 2015.[214] Skyrmions are topological structure in chiral bulk magnets. They are formed by vortex-like magnetic spins and stabilized by Dzyaloshinskii–Moriya interactions. Skyrmions were also found to exist in nonconductive multiferroic Cu2O/SeO3.[215,216] These magnetoelectric skyrmions may be used for the novel electric field controlled spintronic devices. In the experimental studies of skyrmions, spin-polarized STM has been used to visualize and precisely create and destroy the skyrmions by applying local electric field at cryogenic temper-ature.[217,218] While MFM is the technique other than Lorentz TEM to directly visualize micro-to-nanoscale skyrmions, such as their formation and variation under magnetic field at room temperature or cryogenic temperature.[219,220]

In addition to magnetoelectric coupling, multiferroic materials also exhibit piezoelectricity and are sensitive to external strain. Seidel’s group has reported a series of studies on the nanomechanical and electromechanical coupling of morphotropic lead-free strained BFO films using various SPM techniques. The local mechanical force applied by SPM tip

leads to phase transition from tetragonal-like to rhombohedral-like phase in strained BFO film, during which the mechan-ical softening also occurred. The structural phase transition reversal was realized via heating or applying electric field.[221] In addition, via metal doping, both mechanical stiffening (Ca doped) and softening (La doped) occurred during the phase transition.[222] To unravel the dynamic phase transition pro-cesses, DART-PFM was applied to track the shift of contact resonance frequency, which is affected by the elasticity variation of BFO during the phase transition. By applying proper model of cantilever dynamics and tip–sample contact mechanics, the reduced elastic modulus variation can be quantified.[223] Alternatively, force–distance curves are used to quantity the elasticity. In cooperation with C-AFM, morphology, stiffness and electronic conductivity can be correlated in morphotropic BFO. Better conductivity was found at the structural phase boundaries.[224] Furthermore, electric bias induced correlated ferroelectric and phase switching were investigated by PFM,[225] and the strength of EMC was found to be dependent of the thickness of BFO film.[226] On the other hand, stress-induced ferroelectric switching kinetics of PZT film have been studied using the similar method.[227]

4.1.3. Nonferroelectric Oxide Materials

Electric filed can induce, align, or reorient dielectric dipoles. Therefore, all dielectric materials should in principle response to the electrical field applied through the SPM probe. For example, the deformation in piezoelectric oxides is caused by the EMC, whereas the deformation in some other oxides, for instance TiO2, NiO, and VO2, is caused by ECC associated mechanisms. Therefore, the responses to electric field in oxide materials can be very different and need to be studied using appropriate techniques. ESM is one of the main techniques to characterize the electrochemically induced deformation in dielectric materials. The principle of ESM is based on the intrinsic relationship between the molar volume change and bias-induced ionic movement. Various techniques developed in PFM, such as PFS and SS-PFM can also be applied to ESM, namely, EFS or SS-ESM.

Certain oxide thin films are potential candidates for resis-tive random-access memory (RRAM) that has the advantages of low power consumption and good scalability. ZnO is one of the promising candidates showing controllable resistive switching (RS) characteristic (Figure 4iii), which is a fundamental requirement for RRAM. The concomitant properties including RS, photoconductivity, piezoelectric, ferroelectric-like, ferro-magnetic and photoelectrochemical properties make ZnO an appealing material in the applications of spintronics, microelec-tronics and optoelectronics.[228] We have applied various SPM techniques to characterize the ZnO-based materials to study the charge activity, oxygen vacancy formation, piezoelectric and ferroelectric-like properties (Figure 6).[35–40,144,229,230] ZnO is a typical n-type semiconductor with bandgap of 3.37 eV, but it can also be p-type because of Zn vacancies formed by hydrothermal growth method.[228] Pure ZnO in the form of either thin film or nanorod demonstrates clear RS based on the I–V spectroscopy measured by C-AFM.[39,40] Polarization switching can occur

Adv. Mater. 2018, 30, 1803064



when ZnO is switched to the high resistance state (HRS) (Figure 6i) because at the low resistance state (LRS) free charges may strongly screen the polarization. Oxygen partial pressure (PO2) during sample fabrication, especially for PLD processes, is usually used to control the amounts of oxygen vacancy (VO). Deficient oxygen environment usually creates more VO in ZnO, which may act as domain inversion center and assist the polari-zation reversal. Our PFM measurements on samples fabricated by PLD in high or low PO2 confirmed this observation. On the other hand, the observed charging state of ZnO film is affected by the electric poling method (sample or tip biased) and atmos-phere (H2O or O2). Poling induced surface charge can easily be removed by contact mode scans with grounded tip, leaving the injected charge and polarization charge stored in the sample. The actual charge storage ability of ZnO can easily be character-ized with KPFM (Figure 6ii), and only −10 Vdc sample bias (i.e., HRS) led to dramatic increase in surface potential in ambient air.[35,144]

ZnO nanorods mostly show unipolar RS with much higher set/reset voltage. The −10 (+10) Vdc sample bias–induced LRS (HRS), which was opposite to the case of ZnO thin film.[39] Increasing aspect ratio (length/diameter) of the nanorod reduced the set/reset voltages and increased the piezoresponse due to size effect. Similar to that in ZnO thin film, more num-bers of Vo enhanced the piezoresponse and facilitated the

polarization switching (or reorientation). The ferroelectric-like switching of pristine ZnO nanorod showed strong positive imprint that is greatly reduced when ZnO is poled by high DC bias. Adsorbates of water and oxygen molecules in air affected the screening of the polarization charges. By eliminating these adsorbates, piezoresponse and switchable polarization were increased.

Metal doping can modulate the electronic properties of ZnO. The doped Cu atoms can work as electron traps in ZnO, shifting the Fermi level down, and resulting in an increase in resistivity and reduction in current leakage in ZnO films. We hence primarily studied the ZnO:Cu thin films. The resistivity of ZnO:Cu film is positively related to the Cu concentration. Using KPFM method, we found that with proper amount of metal doping (8 at%), the long-term bipolar charge storage ability (polarization charges and/or injected charges) of ZnO:Cu was improved significantly because of the reduced Schottky barrier height or formation of Ohmic contact at the tip–sample junction (Figure 6ii).[35,144] Meanwhile VO helped effectively trap electrons injected from the tip, while metal doping led to better hole trapping phenomenon compared to that in the pure ZnO film. We also found that ZnO:Cu film exhibited multifer-roic behaviors.[229] The ferroelectric domains and ferromagnetic domains were observed using PFM and MFM, respectively (Figure 6iii).[58] In terms of resistive switching, ZnO:Cu thin

Adv. Mater. 2018, 30, 1803064

Figure 6. PFM/KPFM/MFM measurements on ZnO-based thin films, showing the ferroelectric-like and ferromagnetic behavior of the ZnO films. i) PFM measurements on ZnO:Cu (2%) film (deposited at oxygen partial pressure of 2 × 10−4 Torr), a–d) amplitude images, and e–h) phase images of the sample after poling process with 0 V → 15 V → −15 V → 18 V in the middle of 0.5 × 0.5 µm2 area of the sample. ii) Surface potential images immediate after application of the DC biases for: a) undoped ZnO, b) ZnO doped with Co, c) ZnO doped with Cu, d) ZnO doped with Cu and Co. e) The variation of the surface potential as function of poling size (measured at the arrow location) for all the samples. f ) Schematic diagram of flat band structure, also showing the location of the Fermi level for different samples. g) UPS results data for Pt, ZnO:Cu, ZnO:Co, and ZnO:Cu:Co samples. iii) The topographic AFM image of ZnO:Cu (8%) film a); b) its stripe-like magnetic domain structures revealed by MFM imaging; c) MFM image after a 45° rotation. iv) a) The phase loop and “butterfly-shape” amplitude loop obtained by PFS technique; and b) piezoresponse loop calculated by using PR = A × cos(φ) for the same ZnO:Cu thin film sample as in (iii). i,iv) Reproduced with permission.[38] Copyright 2017, Elsevier. ii) Reproduced with permission.[144] Copyright 2012, American Chemical Society. iii) Reproduced with permission.[229] Copyright 2011, Wiley-VCH.



film showed bipolar RS with asymmetric and nonlinear I–V curves (Figure 4iii). The set/reset voltages were increased due to the higher resistivity of the doped film. At low bias, the con-duction behavior is dominated by space-charge–limited current and Ohmic conduction for the set and reset curve respectively, whereas at high bias, it is dominated by Schottky emission. The RS can only be retained by slowly applied high electric bias, which provides sufficient time to form stable resistive states.[37] Similar to the pure ZnO film, polarization switching is directly associated with RS behavior, and only occurs at HRS of ZnO:Cu film. Higher Cu concentration leads to increased piezoelectric constant, while polarization switching is enhanced by maintaining sufficient VO and Cu concentrations. PFS loops showed positive imprint, especially for higher Cu concentrated ZnO films (Figure 6iv). Such imprint could approve the exist-ence of build-in field in ZnO:Cu that might be caused by polar defects or injected charges, which affected both the resistive and polarization switching.[38]

In addition, we have also investigated the PLD fabricated NiO[161] and TiO2

[159,160,162] thin films for potential application as RRAM. The ionic activity, charging effect and electronic con-ductivity of these oxides were studied using ESM, KPFM, and C-AFM techniques, respectively (Figure 5ii,iii).[159–162] TiO2 is an ionic–electronic semiconductor. We found that it regenerated and diminished VO based on redox reactions. Rearranging and clustering of VO led to the formation and disruption of TinO2n−1 (Magneli phases) filaments that were responsible for the bipolar RS behavior. ESM demonstrated the DC bias induced varia-tion of ionic activity due to the changes of the molar volume in TiO2. Low concentration of VO usually caused negligible ESM responses and RS behaviors. For the vertical structure, VO-based filament formation through the film thickness domi-nated the mechanism of RS, while interface-based mecha-nism dominated the RS of lateral structured TiO2 film. For the planer RS cell, with closer tip–electrode distance, I–V curve measurements with high voltage caused severe morphology change and show bipolar RS. With further increased tip–elec-trode distance, highly localized nanobumps were formed and unipolar RS behavior emerges. Morphology deformation indi-cated the irreversible electrochemical reaction when DC bias was higher than 6 V. Environment also played a critical role for the bias-induced electrochemical reaction and RS behavior in TiO2. Based on the study of planer RS cell, moisture in ambient air was a prerequisite for electrochemical reactions and thus also for the RS of TiO2. For vertical TiO2 cell, oxygen reduced the set/reset voltages, and increased the maximum current for resistive switching, whereas moisture had the opposite effect.

To further characterize the ionic and electrochemical induced phenomena, the first-order reverse curve (FORC) method was employed (Figure 4ii).[231] This method can be applied to either I–V curve or ESM measurements (denoted as FORC-IV or FORC-ESM). During the FORC measurement, a bias waveform that consists of a sequence of triangular pulses with increasing amplitude is applied to either the SPM tip or the sample. The FORC-IV measures the current response, while FORC-ESM measures the deformation and phase changes induced by the gradually increased electric bias. At low bias, the responses are linear and nonhysteretic. At sufficiently high bias, the chemical or physical states of the samples are altered and ionic processes

can be activated, therefore, the response curve may become hys-teretic. The evolution of the loops area demonstrates the onset and dynamics of the ionic processes based on the I–V curves or ESM measurements. We have applied FORC-IV technique on TiO2 and NiO thin film samples,[159–162] and the results have demonstrated the strong correlation between ionic processes and RS behaviors in those materials.

4.2. Energy Materials

4.2.1. Li-Ion Battery

Li-ion battery is the most commonly used energy generation/storage system. Typical components in LIB include electrodes (anode and cathode) and electrolyte (solid or liquid). Our researches in this area are mainly focused on all-solid-state full battery (SSFB), and single layer or nanoparticles of the electrode materials. SPM techniques, including ESM, KPFM, BE-ESM, c-AFM, and AM–FM, were applied to characterize the bias-induced deformation, changes of conductivity and elas-ticity of SSFB (Figure 7i) and electrodes due to the electrochem-ical charge/discharge processes.[73,155,157,158,232,233,237–239]

Zhu et al. has observed clear phase changes with the applica-tion of electric bias in SSFB (Figure 7ii).[232] The phase image showed uniform contrast for the as-deposited anode film (TiO2), but a new phase (green color) was visualized from the phase image after the battery was polarized by +3 V bias, and disappeared when the battery was polarized by −3 V bias. The emergence and disappearance of the new phase repeated in the subsequent bias cycles, which were associated with the Li ions diffusion under the different polarities of the electric bias. On the other hand, the significantly different CPD obtained by KPFM on as-deposited and polarized TiO2 anode film in SSFB also indicated the formation of the new phase due to the insertion of Li ions under electric bias (Figure 7iii).[233] Besides SSFB, single-layer cathode films, including LiCoO2,[70,234–236] LiMn2O4,[72] and others, were also extensively characterized by SPM techniques. Our works were mainly focused on layered-oxide cathode materials.[73,157,158,237,238] One of the works was to characterize the as-deposited and cycled LiNi1/3Co1/3Mn1/3O2 film using BE-ESM and C-AFM techniques.[73] It showed that both Q-factor and current decrease with the increase in charging/discharging cycle numbers, with the same trend as the discharge capacity of the battery materials. These measure-ments have established strong links among ionic diffusion, conductivity, surface mechanical properties, and capacity fading for such material.

BE-ESM amplitude (A) can be related to the diffusion coef-ficient (D) by[70,72,236–238]

2 1 acAV Dν βη ω

( )= + (2)

where ν is Poisson’s ratio; β is the Vegard coefficient, which describes the linear relationship between the lithium-ion con-centration and the volume of the material for a continuous solid solution, with the value equals to 0.02349;[70] Vac is the drive amplitude; ω is the frequency of the applied electric

Adv. Mater. 2018, 30, 1803064



field; η is defined as a linear relationship between the poten-tial at the tip and the concentration field of mobile ions,[72] which can be determined by Butler-Volmer voltage activation kinetics.[236] When the ESM tip is taken as a reference, η can be set to equal to the applied voltage. According to Equation (2), ESM amplitude image can be used to extract the local dif-fusion coefficient, and this was done for Li-rich cathode film Li1.2Co0.13Ni0.13Mn0.54O2 (Figure 8i).[158,237,238] The diffusion coefficient determined from BE-ESM image is in the same order of the macroscopic electrochemical measurements. Moreover, multiple BE-ESM scans were used to simulate the effects of numbers of bias cycles on diffusion coefficient.[237] By combining BE-ESM with temperature stage, Yang et al. have derived the activation energy for Li-ion diffusion in Li-rich cathode film at different temperatures (Figure 8ii).[158] In addition, the conductivity of the cathode and the effects of grain boundaries were studied by using C-AFM (Figure 8iii).[238] To study what really changed in the Li-rich cathode film during the charge/discharge cycles, Yang et al. manufactured full-cell batteries with Li-rich material as the cathode. The charged-only and charged/discharged cells were disassembled, and the cathode layer was taken for ESM, C-AFM, and AM–FM char-acterizations.[157] Comparing with the as-deposited cathode film, the surface roughness, current and elastic modulus of the

charged film were deceased. When the cathode film was dis-charged, the roughness was increased, current and elastic mod-ulus were only partially recovered compared to that of the as-deposited film. Those results confirm the irreversible changes due to the charging/discharging cycles in the cathode material and are correlated well with the macroscopic capacity decay during the first cycle in the Li-rich cathode material.

In most industrial applications, nanoparticles are the fun-damental elements for electrodes in Li-ion batteries. Because of the small size and fixation issues, the electrical measure-ment of nanoparticles using SPM is challenging. Li et al. has applied point electric bias on the nanoparticles followed by tapping mode AFM to image the evolution of the topog-raphy of free-stand isolated particle of Li-rich cathode mate-rial (Figure 8iv).[155,239] At the same time, point I–V curves were measured to characterize the dynamic electrochemical reactions, and AM–FM was applied to study the electrically induced changes of the elasticity in the nanoparticles. It was observed that, in ambient condition, the nanoparticles were fractured by a critically large positive bias (Figure 8iv), but such fracture was not observed in water-free environments such as synthetic air or argon. Thus, when critical amounts of Li ions are extracted out of the particles due to the electrochemical reaction facilitated by water, the original structure cannot be

Adv. Mater. 2018, 30, 1803064

Figure 7. i) a) Schematic of the in situ SPM measurement in 1 × 1 µm2 area (scanned by SPM with conductive tip) on the anode surface of an all-solid-state battery, b) FIB cross-sectional image of the thin film solid state battery, and c) cyclic electrical signal applied to the battery anode surface, also showing the SPM scan number. ii) In situ SPM images of thin film anode surface in all-solid-state battery polarized by a cycled bias. From left to right column: height images, deflection images and phase images: a) 1st scan, b) 2nd scan, c) 4th scan, d) 6th scan, and e) 8th scan. iii) Distribution histograms of CPD measured by KPFM on: a) a TiO2 anode film in the all-solid-state battery as shown in (b), and b) a single-layer TiO2 film during the first positive/negative polarization cycle. i,ii) Reproduced with permission.[232] Copyright 2012, Elsevier. iii) Reproduced with permission.[233] Copyright 2012, AIP Publishing.



retained, which will cause fracture of particles. I–V curve meas-urements showed that the conductivity of nanoparticles had a pulse-type current profile under a constant voltage, which was explained by DFT calculations.[239] The high current peaks are caused by the formation of conductive compounds of LiOH due to the Li ions reacting with the water molecules, and fol-lowed by immediate formation of nonconductive compound of Li2O by LiOH reacting with Li ions, which leads to low current. Moreover, ESM loops can also be measured on single particles. The loop opening as a function of applied voltage can be used to determine the onset voltage of the electrochemical reaction activation for the electrode materials. For Li-rich cathode, this

activation voltage was found to be ≈2.38 V, which agreed well with our ESM spectroscopy relaxation measurements.

4.2.2. Other Types of Battery Materials

Besides Li-rich cathode materials, ESM was also applied in many other systems, such as the solid electrolyte Sm-doped ceria,[65] LiMn2O4 electrode (simulation and experiment),[68] and micro- and nanocrystalline lithium iron phosphate LiFePO4,[66] to study the space charge, ionic diffusion, energy dissipation, and more. Although Li-ion batteries are now the main stream

Adv. Mater. 2018, 30, 1803064

Figure 8. i) BE-ESM images (with 3 Vac bias) of Li-rich cathode film: a) resonance frequency, b) resonance amplitude, c) Q-factor images, d) calculated diffusion coefficient map. All images are obtained with a pixel density of 100 × 100 over 0.8 × 0.8 µm2 area. i) Reproduced with permission.[237] Copyright 2015, The Royal Society of Chemistry. ii) a) DRAT-ESM amplitude image (600 × 600 nm2) measured at 55 °C, b) calculated diffusion coefficient map from image (a), c) amplitude profile at selected line with increasing temperatures from 25 to 65 °C, and d) Arrhenius plot of the diffusion coefficients as a function of the inverse of temperature for selected grains. ii) Reproduced with permission.[158] Copyright 2017, American Chemical Society. iii) I–V curves at different locations of Li-rich cathode film: a) AFM height image of 1.1 × 1.1 µm2 where the center of the four selected grains are marked for current measurements (d). b) current as a function of time at the four locations marked in (a). c) AFM height image of an area of 0.5 × 0.5 µm2 where eight selected points are marked (A–F) for current measurements. d) Current as a function of time at the eight locations marked in (iii-c). iii) Reproduced with permis-sion.[238] Copyright 2016, The Royal Society of Chemistry. iv) Multifrequency-AFM images of pristine Li-rich particles laid on a Pt-substrate: a) 3D image of contact stiffness kc by AM–FM and b) bimodal-AM amplitude2 (A2) image. Both images were obtained by scanning under a repulsive force regime (phase1 < 90°). Both images were mapped onto the topographic image. The color scale of (b) was inverted so that kc and A2 images show nearly identical contrasts. The central flat regions of the particles were used in the analysis. c) The pristine Li-rich particle under bias of 2 V, and d) the same particle under the bias of 5.5 V, showing the fracture of the particle. iv) Reproduced with permission.[155] Copyright 2015, The Royal Society of Chemistry.



in the battery market, new designs of battery structures and compositions have been consistently developed, for example, potassium (K)-ion batteries and sodium (Na)-ion batteries are popular candidates mainly due to the abundance and low cost of the resources.[240,241] However, researchers are still seeking for the suitable electrodes and electrolytes that allow smooth intercalation and transportation of K ions or Na ions. The previ-ously mentioned SPM techniques can also be used to probe the ionic activity in these ion battery materials.

4.2.3. Inorganic–Organic Hybrid Perovskite (IOHP) Solar Cells

Many studies have been devoted to IOHP solar cells using high-resolution SPM techniques, mainly C-AFM, pc-AFM, and KPFM, to explore the intrinsic electronic difference between grain interiors and grain boundaries, and the local effects from doping at grain boundaries and interface treatments. Some examples were highlighted to show the beneficial roles of the grain boundaries in various hybrid perovskites.[242,243] KPFM measurements immediately after applying DC bias showed that ionic defects migration could be reduced by incorporating some amount of PbI2, so that the hysteresis between forward and reverse scan was reduced and the device performance could be improved.[244] Many more works have used KPFM to examine the work function and band bending,[245,246] photo-response,[247,248] degradation or passivation of the grain bounda-ries,[249–251] and to explain the macroscopic device behaviors after doping or treatments to the hybrid perovskites. On the other hand, C-AFM and pc-AFM have been used to map the grain boundary conductivity in dark or under illumination of ZnO nano- and microstructured platelets. Grain boundaries showed higher conductivity than that of grain interior in dark and exhibited largely enhanced photoconductivity.[252]

Another interesting research area is on the ferroelectric properties of IOHP. Modeling works have demonstrated the existence of ferroelectricity in different types of IOHP primarily originated from molecular dipoles,[253,254] and its positive role on facilitating the charge separation and bandgap reduction in IOHP.[255] However, the experimental observations are quite controversial mainly due to the large leakage and degradation of IOHP under electric field during the ferroelectric measure-ments. Good news is that ferroelasticity of IOHP has been proved by solid evidences,[166,256,257] which makes strain engi-neering a potential method to further tune the properties of IOHP.[258,259] Using PFM, some recent works also reported the effects from ordered ferroelectric domains,[260,261] and opti-cally induced polarization switching of IOHP.[262] A recent interesting study also showed that the single-crystalline CH3NH3PbI3 actually possesses alternating polar and non-polar domains by using PFM, and this has regulated the photo-current measured by pc-AFM.[263]

4.3. Biological and Organic Materials

Morphological, mechanical, and electromechanical coupling properties are important for proper functions of biological systems. Without sophisticated sample preparation, SPM

techniques can concomitantly provide information of hier-archical structure, nanoscale elasticity mapping, and EMC phenomena.[75,78,117,264–266]

4.3.1. Seashells

Seashells have hard and stiff structures to protect the soft organ-isms. They consist of approximately 5 wt% of soft organic pro-teins, and 95% of brittle minerals (calcium carbonate, CaCO3). Compared to the pure CaCO3, seashells, with a small amount of organic phase, demonstrate orders of increment of both toughness and strength. It has always been a challenge to fab-ricate artificial materials having both high toughness and strength, thus seashells can be good demonstrations from the nature. Using AFM and SEM, we have observed 3–4 levels of hierarchical structures of seashells. Nanograins embedded in an organic matrix are the smallest feature observed using AFM. The hierarchical structure has been reported to be one of the important mechanisms for the superior mechanical properties of the seashells. Such structure is more tolerant to fracture and hinders crack propagation.[267] In addition, the universal intrinsic EMC of biomaterials might also be an important mechanism in the extraordinary mechanical properties of seashells.

Piezoelectricity has been observed in a variety of biological systems, including bones, teeth, wood, crab, lobster shells, human dentine, collagens and many others.[61,110] It has been found that the piezoelectricity was closely related to bone remodeling and regeneration.[268] How such physiologically generated electric field is related to mechanical properties on molecular, cellular, and tissue levels is still unknown but intriguing to be investigated.[269] Using PFM-based techniques, we have found that different types of seashells demonstrated piezoelectricity and ferroelectricity (Figure 9i,ii).[117,264,265] The measured piezoelectric constant was comparable to that of the x-cut quartz. The ferroelectric hysteresis loops obtained from seashells usually showed large imprint, which indicated strong preferential orientation of the biopolymer that could not be easily switched electrically.[117] Such EMC is originated from the biopolymers and protein matrix inside seashells. Greatly reduced piezoresponse was observed when the biopolymers were bleached using NaOH solution or eliminated via heating above their decomposition temperatures. The biopolymers also worked as the adhesive to hold the mineral grains together. Based on the strength of piezoresponse, the location of trans-parent biopolymers can be identified. Seashells always contain small amount of water. When water was removed by drying the sample at 100 °C, the EMC due to the ionic motion were greatly reduced, but both piezoelectric and ferroelectric behaviors were still persisted. To relate mechanical properties to EMC, we have applied flexural stresses to the cross-sectional abalone shell samples.[266] Under tensile stress, the nanograins sunk into the biopolymer matrix and the average piezoresponse was increased. In this case, the polymer chains might be aligned by stress, giving better crystallinity and internal field that led to imprint of ferroelectric loops (Figure 9ii). EMC may also work as an energy dissipation mechanism that converts external mechanical energy into internal electric energy, which may be sensed and consumed by the living organisms. EMC can be an

Adv. Mater. 2018, 30, 1803064



important characteristic for both mechanical strength and bio-physical behaviors of living systems.

On the other hand, with the multilevel hierarchical structure, the mechanical properties of seashells are expected to be highly inhomogeneous. In our work, the high-resolution quantitative elastic modulus mappings were observed by CR-FM.[75] The surface and bulk modulus were mapped using different loading force. The elastic moduli of various types of mineral grains were found to be ranging from 80 to 100 GPa depending on grain shape and observation directions of the seashell.

4.3.2. Collagen Fibers

Collagen is one of the basic building blocks in bone’s hierar-chical structure and provides the template for mineralization of bone. Genetically mutated collagen fibers can cause severe bone disease. Osteogenesis imperfecta (OI) is a type of brittle bone disease that is fatal to human. OI is caused by type I collagen genetic mutation, by which the only α2 chain of het-erotrimer is replaced by α1 chain and form homotrimer in tropocollagen molecules.[270] Such mutation leads to dramatic degradation of overall bone mechanical properties. To gain a deeper understanding of the disease, studies of the diseased

pristine collagen at both macroscale and nanoscale are neces-sary. Human OI can be represented by a mouse model (oim/oim). We have hence adopted various SPM techniques to inves-tigate the nanoscale differences in structure and mechanical properties between the healthy (+/+) and oim/oim collagen fibers.[78] AFM was used to observe morphological details. Both types of fibers had periodic substructures, but +/+ fibers had larger D-spacing and heterogeneity and larger diameter than those of oim/oim fibers (Figure 9iii). AM–FM was used to quantify the stiffness variation along a single collagen fiber. +/+ fibers had periodic variation of stiffness that was closely related to the fiber morphology, while oim/oim fiber had random stiff-ness variation and the spots with low stiffness. These spots may reflect the internal structural defects that are not visible from the morphology. The +/+ fibers were generally stiffer than oim/oim fibers (Figure 9iii). Furthermore, PFM was used to investi-gate the EMC of individual collagen fibers. When out-of-plane electric field was applied, the fibers showed more deformation in the in-plane direction, which indicates preferred shear defor-mation. Similar to the case of stiffness variations, +/+ fibers have much stronger and more ordered piezoresponse than that of oim/oim ones. Both types of fibers also exhibit ferroelectric behaviors, but the dipoles in +/+ fibers can be switched more easily with smaller coercive bias and imprint.

Adv. Mater. 2018, 30, 1803064

Figure 9. i) AFM/DART-v-PFM/DART-l-PFM images of nacre: a) DART-v-PFM amplitude, b) DART-v-PFM phase, c) DART-l-PFM amplitude, and d) DART-l-PFM phase images. All images were observed from the cross-sectional nacre surface (800 × 400 nm2, 512 × 256 pixels). i) Reproduced with permission.[117] Copyright 2013, AIP Publishing. ii) PFS loops of cross-sectional nacre under different flexural stresses: a) amplitude loops, b) phase offset loops, notice ≈180° phase angle changes, and c) PR hysteresis loops. The annotations “C,” “T,” and “M” indicate compression, tension and zero-stress positions, respectively. ii) Reproduced with permission.[266] Copyright 2013, Elsevier. iii) AFM images of: a) amplitude of +/+ collagen fibers, b) oim/oim collagen fibers. Scan size is 2 × 2 µm2 with 512 × 512 pixels. c) Box chart of the D-spacing of the +/+ and oim/oim collagen fibers with average, deviations, and the data extremes. The dashed line corresponds to the theoretical value of D-spacing of 67 nm. d) AM–FM stiffness mapping of the +/+ collagen fibers, and e) AM–FM stiffness mapping of oim/oim collagen fibers. Scan size is 0.5 × 0.5 µm2 with 256 × 256 pixels. f) Histograms of the stiffness images, showing the stiffness comparison between the +/+ and oim/oim collagen fibers. iii) Reproduced with permission.[78] Copyright 2016, Elsevier. iv) Calculated PR hysteresis loop of NUS-6 (Hf) MOF crystal. The inset shows crystal structure of NUS-6 featured by micropores (orange spheres) and mesopores (yellow spheres). iv) Reproduced with permission.[199] Copyright 2017, The Royal Society of Chemistry.



On the other hand, the multifrequency SPM has been applied in a rodent model of sepsis to investigate the effects of systemic sepsis and inflammation on bone strength.[156] No significant differences have been observed from microCT, histomorphometry, bone morphology and mass density between the sham and sepsis treated mice’s bone. However, AM–FM tests have demonstrated clear reduction of the elastic modulus in the collagen in 24 h probably due to the loss of intermolecular and intrafibrillar cross-links, and reduced elastic modulus of mineral in 96 h due to the loss of highly mobilized hydroxyapatite of sepsis samples comparing to sham samples. In this work, AM–FM demonstrated its uniqueness and showed that such mechanical property deterioration is a result of altered biochemical properties of bone, as opposed to bone turnover driven loss of mineralization, and thus provides valuable clinical implications.[156]

In these works, SPM techniques have realized the ultrahigh-resolution mapping of morphology, stiffness and EMC of col-lagen fibers and even revealed subfibril features. These obser-vations have provided deeper insights into the understanding of the relationship between the structure, mechanical property and mineralization of +/+ and oim/oim collagen fibers at fibril and subfibril level. It has also implicated that the collagen fiber initiated mechanical failure of the bone. Therefore, the multi-field characterizations can give us more comprehensive under-standing of the material properties and behaviors in biological systems.

Besides calcified tissues like bone and seashells, many researchers have also investigated the EMC in soft biological materials, such as arterial elastin,[271] and aortic wall.[116] It fur-ther demonstrates the EMC as a universal feature in biological systems, based on which bioengineering materials or devices can be developed, for example piezoelectric scaffold was fabri-cated to facilitate stem cell differentiation.[272]

4.3.3. Organic and Molecular Ferroelectrics

The abovementioned ferroelectrics crystals and films are mostly hard and stiff inorganic materials. Organic ferroelectrics (OFs), as another family of ferroelectrics, demonstrate struc-tural flexibility and are environmental friendly. OFs are mainly targeted for flexible electronics and biomedical applications. Some examples of OFs are PVDF, KH2PO4 (KDP), and tetrathi-afulvalene (TTF) complexes. OFs are based on noncovalent molecules, and their ferroelectricity arises from distinctive mechanisms, such as charge transfer, dipole reorientation, and structure-phase transformation. However, the major drawbacks of OFs are the low spontaneous polarization, low phase tran-sition temperature and small dielectric constant. The princi-ples, advantages and limitations of organic ferroelectrics have been comprehensively reviewed by Horiuchi and Tokura.[109] Croconic acid and diisopropylammonium bromide (DIPAB) are two types of molecular ferroelectrics showing large spon-taneous polarization (≈23 µC cm−2),[273,274] thus have attracted much attention in this field. The in-plane domain switching of DIPAB and the stable charged domain walls were reported by Lu et al.[275] The recent developments of molecular ferroelec-trics have been reviewed by Li and co-workers.[276] Similar to

that of the inorganic ferroelectrics, the domain structure and polarization switching of organic ferroelectric materials can also be characterized by PFM-based techniques.

4.4. Supramolecular Materials

MOF nanocrystals possess an intriguing supramolecular design and are easily synthesized, hence can be perfect hosts for new ferroelectrics and multiferroics. They can be self-assembled into 3D crystals with large voids extended along multiple axes. Our recent results showed that the UiO-66(Hf)-type MOFs have higher elastic modulus (46–104 GPa) than that of UiO-66(Zr)-type MOFs (34–100 GPa), whereas the zinc/copper-based MOFs had lowest modulus (3–10 GPa). We also demonstrated that the mechanical properties of MOFs could be tuned by adjusting the chemical functionalities of the ligands or using different metal nodes.[79] The elasticity could also be correlated with the structural-failure of MOFs induced by multiple-cycles of CO2 adsorption and desorption.[277] The PFM measurements con-firmed the piezoelectric and ferroelectric behaviors in NUS-6(Hf) and NUS-6(Zr) nanocrystals, and NUS-6(Hf) MOFs showed better ferroelectricity than that of NUS-6(Zr) (Figure 9iv). These works can provide guidelines on designing novel MOF-based ferroelectric materials in the future.

5. Future Perspective and Concluding Remarks

We have reviewed the MCP in functional and structural materials, and various SPM techniques that can be applied to characterize these coupling phenomena. There are vast number of literatures related to the principles and applica-tions of SPM techniques to study the MCP, and our focuses in this review are on the local electronic, mechanical, chemical, optical, and environmental coupling phenomena. Studies of the local MCP can guide us to understand many functional properties of the advanced materials at their molecular or microstructural levels. Such understandings are fundamentals and can provide guideline for designing innovative materials and devices. The SPM techniques are no doubt the powerful and ideal tools to study the local MCP. The continuous development and enhancement of various SPM techniques will provide more opportunities to these studies. In the future, comprehensive understandings of the MCP in functional and structural mate-rials will rely on the collaborative works and progresses from experiments, computation, and newly developed big-deep-smart data analysis for SPM techniques, and the development of novel SPM techniques. From this point of view, artificial intelligence for SPM image analysis may be one of the new directions. The future will undoubtedly see a much broad spectrum of novel developments in understanding the MCP from local to global levels and more advanced characterization tools and techniques.

AcknowledgementsThese studies were supported by National University of Singapore (NUS). The authors greatly thank the financial support from Ministry of

Adv. Mater. 2018, 30, 1803064



Education, Singapore, through NUS on several Academic Research fundings (AcRF) (Grant Nos. R-265-000-305-112, R-265-000-257-112, R-265-000-190-112, R-265-000-495-112, R-265-000-532-112, and R-265- 000-596-112). The authors also thank the current and previous postdoctoral fellows and graduate students in the group who have contributed to the works described in this paper. The authors also would like to thank all of their local and international collaborators.

Conflict of InterestThe authors declare no conflict of interest.

Keywordsbiomaterials, energy materials, multifield coupling, oxide materials, scanning probe microscopy techniques

Received: May 13, 2018Revised: July 22, 2018

Published online: October 10, 2018

[1] Y. Qing, W. Yubo, T. Mingwei, X. Pengfei, X. Yingke, L. Xu, Semi-cond. Sci. Technol. 2017, 32, 083004.

[2] Q.-H. Qin, Q.-S. Yang, Effective Coupling Properties of Heteroge-neous Materials, Springer, Berlin, Germany 2009.

[3] P. Mingzeng, Z. Yan, L. Yudong, S. Ming, Z. Junyi, W. Z. Lin, Adv. Mater. 2014, 26, 6767.

[4] H. Xu, Y. Pei, F. Li, D. Fang, J. Mech. Phys. Solids 2018, 114, 143.[5] E. Carrera, M. Boscolo, A. Robaldo, Arch. Comput. Methods Eng.

2007, 14, 383.[6] C. M. Hamel, F. Cui, S. A. Chester, Int. J. Numer. Methods Eng.

2017, 111, 447.[7] L. Yudong, G. Junmeng, Y. Aifang, Z. Yang, K. Jinzong, Z. Ke,

W. Rongmei, Z. Yan, Z. Junyi, W. Z. Lin, Adv. Mater. 2018, 30, 1704524.

[8] T. Jia, Z. Fan, J. Yao, C. Liu, Y. Li, J. Yu, B. Fu, H. Zhao, M. Osada, E. N. Esfahani, Y. Yang, Y. Wang, J.-Y. Li, H. Kimura, Z. Cheng, ACS Appl. Mater. Interfaces 2018, 10, 20712.

[9] P. Lin, X. Yan, F. Li, J. Du, J. Meng, Y. Zhang, Adv. Mater. Interfaces 2017, 4, 1600842.

[10] C. Liu, H. Zhao, Z. Ma, Y. Qiao, K. Hong, Z. Ren, J. Zhang, Y. Pei, L. Ren, Rev. Sci. Instrum. 2018, 89, 025112.

[11] Y.-T. Cheng, M. W. Verbrugge, J. Power Sources 2009, 190, 453.[12] T.-Y. Zhang, M. Zhao, P. Tong, Adv. Appl. Mech. 2002, 38, 147.[13] T. Y. Zhang, C. F. Gao, Theor. Appl. Fract. Mech. 2004, 41, 339.[14] K. Sieradzki, R. C. Newman, J. Phys. Chem. Solids 1987, 48, 1101.[15] A. Sawa, Mater. Today 2008, 11, 28.[16] S. V. Kalinin, A. Borisevich, D. Fong, ACS Nano 2012, 6, 10423.[17] J. Liu, G. Cao, Z. Yang, D. Wang, D. Dubois, X. Zhou, G. L. Graff,

L. R. Pederson, J.-G. Zhang, ChemSusChem 2008, 1, 676.[18] S. V. Kalinin, R. Shao, D. A. Bonnell, J. Am. Ceram. Soc. 2005, 88,

1077.[19] A. A. Bokov, Z.-G. Ye, J. Mater. Sci. 2006, 41, 31.[20] Y. Zhang, J. Li, D. Fang, J. Mech. Phys. Solids 2013, 61, 114.[21] T.-Y. Zhang, T. Wang, M. Zhao, Acta Mater. 2003, 51, 4881.[22] W. Yang, F. Fang, M. Tao, Int. J. Solids Struct. 2001, 38, 2203.[23] M. Fiebig, T. Lottermoser, D. Meier, M. Trassin, Nat. Rev. Mater.

2016, 1, 16046.[24] H. Schmid, Ferroelectrics 1994, 162, 317.[25] N. A. Spaldin, M. Fiebig, Science 2005, 309, 391.[26] J. Zhu, K. Zeng, L. Lu, Metall. Mater. Trans. A 2013, 44, 26.

[27] J. Zhu, K. Zeng, L. Lu, J. Solid State Electrochem. 2012, 16, 1877.[28] J. Zhu, K. Zeng, L. Lu, Electrochim Acta 2012, 68, 52.[29] J. Zhu, K. B. Yeap, K. Zeng, L. Lu, Thin Solid Films 2011, 519, 1914.[30] K. Zeng, J. Zhu, Mech. Mater. 2015, 91, 323.[31] K. Y. Zeng, in Handbook of Theoretical and Computationl Nanotech-

nology, Vol. 4 (Eds: M. Rieth, W. Schommers), American Scientific Publishers, Valencia, CA, USA 2006, p. 387.

[32] W. Lü, C. Li, L. Zheng, J. Xiao, W. Lin, Q. Li, X. R. Wang, Z. Huang, S. Zeng, K. Han, W. Zhou, K. Zeng, J. Chen, Ariando, W. Cao, T. Venkatesan, Adv. Mater. 2017, 29, 1606165.

[33] L. M. Kukreja, A. K. Das, P. Misra, Bull. Mater. Sci. 2009, 32, 247.[34] Q. Mao, Z. Ji, J. Xi, J. Phys. D: Appl. Phys. 2010, 43, 395104.[35] M. F. Wong, T. S. Herng, Z. Zhang, K. Zeng, J. Ding,

Appl. Phys. Lett. 2010, 97, 232103.[36] T. S. Herng, A. Kumar, C. S. Ong, Y. P. Feng, Y. H. Lu, K. Y. Zeng,

J. Ding, Sci. Rep. 2012, 2, 587.[37] J. Xiao, T. S. Herng, J. Ding, K. Zeng, J. Alloys Compd. 2017, 709,

535.[38] J. Xiao, T. S. Herng, J. Ding, K. Zeng, Acta Mater. 2017, 123, 394.[39] J. Xiao, W. L. Ong, Z. Guo, G. W. Ho, K. Zeng, ACS Appl. Mater.

Interfaces 2015, 7, 11412.[40] J. Xiao, K. Zeng, L.-M. Wong, S. Wang, J. Mater. Res. 2015, 30,

3431.[41] R. Garcıa, R. Pérez, Surf. Sci. Rep. 2002, 47, 197.[42] R. Garcia, A. S. Paulo, Phys. Rev. B 1999, 60, 7.[43] Á. S. Paulo, R. García, Phys. Rev. B 2001, 64, 193411.[44] G. Haugstad, Atomic Force Microscopy: Understanding Basic Modes

and Advanced Applications, John Wiley & Sons, Inc., Hoboken, NJ, USA 2012.

[45] A. Avila, B. Bhushan, Crit. Rev. Solid State Mater. Sci. 2010, 35, 38.[46] G. Benstetter, A. Hofer, D. Liu, W. Frammelsberger, M. Lanza, in

Conductive Atomic Force Microscopy (Ed: M. Lanza), Wiley-VCH, Weinheim, Germany 2017.

[47] W. Melitz, J. Shen, A. C. Kummel, S. Lee, Surf. Sci. Rep. 2011, 66, 1.[48] Kelvin Probe Force Microscopy: From Single Charge Detection to

Device Characterization (Eds: S. Sadewasser, T. Glatzel), Springer International Publishing, Cham, Switzerland 2018.

[49] G. Paul, Nanotechnology 2001, 12, 485.[50] D. U. Karatay, J. S. Harrison, M. S. Glaz, R. Giridharagopal,

D. S. Ginger, Rev. Sci. Instrum. 2016, 87, 053702.[51] D. Rugar, H. J. Mamin, P. Guethner, S. E. Lambert, J. E. Stern,

I. McFadyen, T. Yogi, J. Appl. Phys. 1990, 68, 1169.[52] F. A. Ferri, M. A. Pereira-da-Silva, E. Marega Jr., in Atomic Force

Microscopy: Imaging, Measuring and Manipulating Surfaces at the Atomic Scale (Ed: V. Bellitto), IntechOpen, London, UK 2012.

[53] A. Majumdar, Annu. Rev. Mater. Sci. 1999, 29, 505.[54] E. Nasr Esfahani, F. Ma, S. Wang, Y. Ou, J. Yang, J. Li, Natl. Sci.

Rev. 2018, 5, 59.[55] G. Séverine, A. Ali, C. Pierre-Olivier, Phys. Status Solidi. A 2015,

212, 477.[56] M. A. O’Connell, A. J. Wain, Anal. Methods 2015, 7, 6983.[57] T. Doi, M. Inaba, H. Tsuchiya, S.-K. Jeong, Y. Iriyama, T. Abe,

Z. Ogumi, J. Power Sources 2008, 180, 539.[58] M. Kang, D. Momotenko, A. Page, D. Perry, P. R. Unwin, Langmuir

2016, 32, 7993.[59] S. Jesse, A. P. Baddorf, S. V. Kalinin, Appl. Phys. Lett. 2006, 88,

062908.[60] S. V. Kalinin, B. J. Rodriguez, S. Jesse, J. Shin, A. P. Baddorf,

P. Gupta, H. Jain, D. B. Williams, A. Gruverman, Microsc. Micro-anal. 2006, 12, 206.

[61] S. V. Kalinin, A. Rar, S. Jesse, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 2006, 53, 2226.

[62] S. V. Kalinin, B. J. Rodriguez, S. Jesse, E. Karapetian, B. Mirman, E. A. Eliseev, A. N. Morozovska, Annu. Rev. Mater. Res. 2007, 37, 189.

Adv. Mater. 2018, 30, 1803064



[63] V. V. Shvartsman, B. Dkhil, A. L. Kholkin, Annu. Rev. Mater. Res. 2013, 43, 423.

[64] J. Li, J.-F. Li, Q. Yu, Q. N. Chen, S. Xie, J. Materiomics 2015, 1, 3.[65] Q. N. Chen, S. B. Adler, J. Li, Appl. Phys. Lett. 2014, 105, 201602.[66] Q. N. Chen, Y. Liu, Y. Liu, S. Xie, G. Cao, J. Li, Appl. Phys. Lett.

2012, 101, 063901.[67] S. Y. Luchkin, K. Romanyuk, M. Ivanov, A. L. Kholkin, J. Appl. Phys.

2015, 118, 072016.[68] H.-Y. Amanieu, H. N. M. Thai, S. Y. Luchkin, D. Rosato,

D. C. Lupascu, M.-A. Keip, J. Schröder, A. L. Kholkin, J. Appl. Phys. 2015, 118, 055101.

[69] K. Romanyuk, C. M. Costa, S. Y. Luchkin, A. L. Kholkin, S. Lanceros-Méndez, Langmuir 2016, 32, 5267.

[70] N. Balke, S. Jesse, A. N. Morozovska, E. Eliseev, D. W. Chung, Y. Kim, L. Adamczyk, R. E. Garcia, N. Dudney, S. V. Kalinin, Nat. Nanotechnol. 2010, 5, 749.

[71] S. V. Kalinin, Y. Kim, A. Kumar, E. Strelcov, N. Balke, T. M. Arruda, S. Jesse, D. Leonard, A. Borisevich, Microsc. Today 2012, 20, 10.

[72] A. N. Morozovska, E. A. Eliseev, N. Balke, S. V. Kalinin, J. Appl. Phys. 2010, 108, 053712.

[73] J. Zhu, L. Lu, K. Zeng, ACS Nano 2013, 7, 1666.[74] B. D. Huey, Annu. Rev. Mater. Res. 2007, 37, 351.[75] T. Li, K. Zeng, Nanoscale 2014, 6, 2177.[76] D. C. Hurley, in Applied Scanning Probe Methods XI: Scanning

Probe Microscopy Techniques, Vol. XI (Eds: B. Bhushan, H. Fuchs), Springer, Heidelberg 2009, p. 97.

[77] D. C. Hurley, in Scanning Probe Microscopy of Functional Mate-rials: Nanoscale Imaging and Spectroscopy (Eds: S. V. Kalinin, A. Gruverman), Springer, New York 2010, p. 95.

[78] T. Li, S.-W. Chang, N. Rodriguez-Florez, M. J. Buehler, S. Shefelbine, M. Dao, K. Zeng, Biomaterials 2016, 107, 15.

[79] Y. Sun, Z. Hu, D. Zhao, K. Zeng, ACS Appl. Mater. Interfaces 2017, 9, 32202.

[80] G. Chawla, S. D. Solares, Appl. Phys. Lett. 2011, 99, 074103.[81] D. Ebeling, S. D. Solares, Beilstein J. Nanotechnol. 2013, 4, 198.[82] R. Garcia, R. Proksch, Eur. Polym. J. 2013, 49, 1897.[83] M. Kocun, A. Labuda, W. Meinhold, I. Revenko, R. Proksch, ACS

Nano 2017, 11, 10097.[84] C. A. Amo, A. P. Perrino, A. F. Payam, R. Garcia, ACS Nano 2017,

11, 8650.[85] R. Proksch, Appl. Phys. Lett. 2006, 89, 113121.[86] N. Schön, D. C. Gunduz, S. Yu, H. Tempel, R. Schierholz,

F. Hausen, Beilstein J. Nanotechnol. 2018, 9, 1564.[87] C. E. Harvey, E. M. van Schrojenstein Lantman, A. J. G. Mank,

B. M. Weckhuysen, Chem. Commun. 2012, 48, 1742.[88] A. Dazzi, C. B. Prater, Chem. Rev. 2017, 117, 5146.[89] H. Lu, C.-W. Bark, D. Esque de los Ojos, J. Alcala, C. B. Eom,

G. Catalan, A. Gruverman, Science 2012, 336, 59.[90] O. Auciello, J. F. Scott, R. Ramesh, Phys. Today 1998, 51, 22.[91] A. Chanthbouala, A. Crassous, V. Garcia, K. Bouzehouane, S. Fusil,

X. Moya, J. Allibe, B. Dlubak, J. Grollier, S. Xavier, C. Deranlot, A. Moshar, R. Proksch, N. D. Mathur, M. Bibes, A. Barthelemy, Nat. Nantechnol. 2012, 7, 101.

[92] F.-R. Fan, Z.-Q. Tian, Z. Lin Wang, Nano Energy 2012, 1, 328.[93] Y. Wang, Y. Yang, Z. L. Wang, npj Flexible Electron. 2017, 1, 10.[94] O. Caballero-Caleroa, R. D’Agosta, ECS J. Solid State Sci. Technol.

2017, 6, N3065.[95] X. Su, P. Wei, H. Li, W. Liu, Y. Yan, P. Li, C. Su, C. Xie, W. Zhao,

P. Zhai, Q. Zhang, X. Tang, C. Uher, Adv. Mater. 2017, 29, 1602013.[96] B. Parida, S. Iniyan, R. Goic, Renewable Sustainable Energy Rev.

2011, 15, 1625.[97] K. Branker, M. J. M. Pathak, J. M. Pearce, Renewable Sustainable

Energy Rev. 2011, 15, 4470.[98] O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic, A. Kis,

Nat. Nanotechnol. 2013, 8, 497.

[99] Z. Yin, H. Li, H. Li, L. Jiang, Y. Shi, Y. Sun, G. Lu, Q. Zhang, X. Chen, H. Zhang, ACS Nano 2012, 6, 74.

[100] A. Pospischil, M. M. Furchi, T. Mueller, Nat. Nanotechnol. 2014, 9, 257.

[101] T. A. Gosavi, S. Manipatruni, S. V. Aradhya, G. E. Rowlands, D. Nikonov, I. A. Young, S. A. Bhave, IEEE Trans. Magn. 2017, 53, 1.

[102] A. Vedyayev, N. Ryzhanova, N. Strelkov, M. Titova, M. Chshiev, B. Rodmacq, S. Auffret, L. Cuchet, L. Nistor, B. Dieny, Phys. Rev. B 2017, 95, 064420.

[103] M. Jamali, J. S. Lee, J. S. Jeong, F. Mahfouzi, Y. Lv, Z. Zhao, B. K. Nikolic, K. A. Mkhoyan, N. Samarth, J.-P. Wang, Nano Lett. 2015, 15, 7126.

[104] S. V. Borisenko, D. V. Evtushinsky, Z. H. Liu, I. Morozov, R. Kappenberger, S. Wurmehl, B. Büchner, A. N. Yaresko, T. K. Kim, M. Hoesch, T. Wolf, N. D. Zhigadlo, Nat. Phys. 2015, 12, 311.

[105] B. Yang, T. Pavel, P. Jaakko, J. Heli, J. Jari, Adv. Mater. 2017, 29, 1700767.

[106] M. F. Wong, K. Zeng, J. Appl. Phys. 2010, 107, 1.[107] M. F. Wong, K. Zeng, J. Am. Ceram. Soc. 2011, 94, 1079.[108] H. Wang, K. Zeng, J. Materiomics 2016, 2, 309.[109] S. Horiuchi, Y. Tokura, Nat. Mater. 2008, 7, 357.[110] S. V. Kalinin, B. J. Rodriguez, J. Shin, S. Jesse, V. Grichko,

T. Thundat, A. P. Baddorf, A. Gruverman, Ultramicroscopy 2006, 106, 334.

[111] E. Fukada, J. Phys. Soc. Jpn. 1955, 10, 149.[112] E. Fukada, I. Yasuda, J. Phys. Soc. Jpn. 1957, 12, 1158.[113] E. Fukada, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 2000, 47,

1277.[114] C. Halperin, S. Mutchnik, A. Agronin, M. Molotskii, P. Urenski,

M. Salai, G. Rosenman, Nano Lett. 2004, 4, 1253.[115] B. J. Rodriguez, S. V. Kalinin, J. Shin, S. Jesse, V. Grichko,

T. Thundat, A. P. Baddorf, A. Gruverman, J. Struct. Biol. 2006, 153, 151.

[116] Y. Liu, Y. Zhang, M.-J. Chow, Q. Chen, J. Li, Phys. Rev. Lett. 2012, 108, 078103.

[117] T. Li, K. Zeng, J. Appl. Phys. 2013, 113, 187202.[118] S. N. Bagriantsev, E. O. Gracheva, P. G. Gallagher, J. Biol. Chem.

2014, 289, 31673.[119] S. V. Kalinin, B. J. Rodriguez, S. Jesse, P. Maksymovych, K. Seal,

M. Nikiforov, A. P. Baddorf, A. L. Kholkin, R. Proksch, Mater. Today 2008, 11, 16.

[120] S. V. Kalinin, S. Jesse, A. Tselev, A. P. Baddorf, N. Balke, ACS Nano 2011, 5, 5683.

[121] K. Yoshikawa, H. Kawasaki, W. Yoshida, T. Irie, K. Konishi, K. Nakano, T. Uto, D. Adachi, M. Kanematsu, H. Uzu, K. Yamamoto, Nat. Energy 2017, 2, 17032.

[122] W. S. Yang, B.-W. Park, E. H. Jung, N. J. Jeon, Y. C. Kim, D. U. Lee, S. S. Shin, J. Seo, E. K. Kim, J. H. Noh, S. I. Seok, Science 2017, 356, 1376.

[123] S. Sadewasser, N. Nicoara, in Kelvin Probe Force Microscopy: From Single Charge Detection to Device Characterization (Eds: S. Sad-ewasser, T. Glatzel), Springer International Publishing, Cham, Switzerland 2018, p. 119.

[124] K. Yasemin,B. A. Aguirre, J. L. Bosse, J. L. Cruz-Campa, D. Zubia, B. D. Huey, Prog. Photovoltaics 2016, 24, 315.

[125] D. C. Coffey, O. G. Reid, D. B. Rodovsky, G. P. Bartholomew, D. S. Ginger, Nano Lett. 2007, 7, 738.

[126] G. E. Eperon, D. Moerman, D. S. Ginger, ACS Nano 2016, 10, 10258.

[127] S. Y. Leblebici, L. Leppert, Y. Li, S. E. Reyes-Lillo, S. Wickenburg, E. Wong, J. Lee, M. Melli, D. Ziegler, D. K. Angell, D. F. Ogletree, P. D. Ashby, F. M. Toma, J. B. Neaton, I. D. Sharp, A. Weber-Bargioni, Nat. Energy 2016, 1, 16093.

[128] R. Berger, A. L. Domanski, S. A. L. Weber, Eur. Polym. J. 2013, 49, 1907.

Adv. Mater. 2018, 30, 1803064



[129] I. Visoly-Fisher, S. R. Cohen, D. Cahen, Appl. Phys. Lett. 2003, 82, 556.

[130] M. Dante, J. Peet, T.-Q. Nguyen, J. Phys. Chem. C 2008, 112, 7241.[131] Y. Yuan, J. Chae, Y. Shao, Q. Wang, Z. Xiao, A. Centrone, J. Huang,

Adv. Energy Mater. 2015, 5, 1500615.[132] K. T. Butler, J. M. Frost, A. Walsh, Energy Environ. Sci. 2015, 8, 838.[133] I. Grinberg, D. V. West, M. Torres, G. Gou, D. M. Stein, L. Wu,

G. Chen, E. M. Gallo, A. R. Akbashev, P. K. Davies, J. E. Spanier, A. M. Rappe, Nature 2013, 503, 509.

[134] H. Matsuo, Y. Noguchi, M. Miyayama, Nat. Commun. 2017, 8, 207.

[135] G. Binnig, C. F. Quate, C. Gerber, Phys. Rev. Lett. 1986, 56, 930.[136] A. Labuda, R. Proksch, Appl. Phys. Lett. 2015, 106, 253103.[137] M. Kocun, A. Labuda, J. Cleveland, N. Geisse, B. Ohler, R. Proksch,

M. Viani, D. Walters, Microsc. Anal. 2014, 28, 21.[138] B. J. Rodriguez, C. Callahan, S. V. Kalinin, R. Proksch, Nanotech-

nology 2007, 18, 475504.[139] S. Jesse, S. V. Kalinin, R. Proksch, A. P. Baddorf, B. J. Rodriguez,

Nanotechnology 2007, 18, 435503.[140] S. Jesse, S. V. Kalinin, J. Phys. D: Appl. Phys. 2011, 44, 464006.[141] S. Jesse, R. K. Vasudevan, L. Collins, E. Strelcov, M. B. Okatan,

A. Belianinov, A. P. Baddorf, R. Proksch, S. V. Kalinin, Annu. Rev. Phys. Chem. 2014, 65, 519.

[142] S. Jesse, A. Kumar, S. V. Kalinin, A. Gannepali, R. Proksch, Microsc. Today 2010, 18, 34.

[143] L. Wang, H. Wang, M. Wagner, Y. Yan, D. S. Jakob, X. G. Xu, Sci. Adv. 2017, 3, e1700255.

[144] A. Kumar, T. S. Herng, K. Zeng, J. Ding, ACS Appl. Mater. Interfaces 2012, 4, 5276.

[145] W. R. Silveira, E. M. Muller, T. N. Ng, D. Dunlap, J. A. Marohn, in Scanning Probe Microscopy: Electrical and Electromechanical Phe-nomena at the Nanoscale, Vol. III (Eds: S. V. Kalinin, A. Gruverman), Springer-Verlag, New York 2007, p. 788.

[146] Y. Yang, X. Zhang, L. Qin, Q. Zeng, X. Qiu, R. Huang, Nat. Commun. 2017, 8, 15173.

[147] S. S. Nonnenmann, X. Chen, D. A. Bonnell, in Scanning Probe Microscopy for Energy Research (Eds: D. A. Bonnell, S. V. Kalinin), World Scientific, Singapore 2013, p. 457.

[148] S. Wu, F. Kienberger, H. Tanbakuchi, in Scanning Probe Microscopy for Energy Research (Eds: D. A. Bonnell, S. V. Kalinin), World Scien-tific, Singapore 2013, p. 481.

[149] O. Amster, F. Stanke, S. Friedman, Y. Yang, S. J. Dixon-Warren, B. Drevniok, Microelectron. Reliab. 2017, 76–77, 214.

[150] T. S. Jones, C. R. Pérez, J. J. Santiago-Avilés, AIP Adv. 2017, 7, 025207.

[151] D. Wu, X. Li, L. Luan, X. Wu, W. Li, M. N. Yogeesh, R. Ghosh, Z. Chu, D. Akinwande, Q. Niu, K. Lai, Proc. Natl. Acad. Sci. USA 2016, 113, 8583.

[152] S. V. Kalinin, D. A. Bonnell, Appl. Phys. Lett. 2001, 78, 1306.[153] S. V. Kalinin, D. A. Bonnell, J. Appl. Phys. 2002, 91, 832.[154] B. Cappella, G. Dietler, Surf. Sci. Rep. 1999, 34, 103.[155] T. Li, B. Song, L. Lu, K. Zeng, Phys. Chem. Chem. Phys. 2015, 17,

10257.[156] Z. A. Puthucheary, Y. Sun, K. Zeng, L. H. Vu, Z. W. Zhang, R. Z. L. Lim,

N. S. Y. Chew, M. E. Cove, Crit. Care Med. 2017, 45, e1254.[157] S. Yang, J. Wu, B. Yan, L. Li, Y. Sun, L. Lu, K. Zeng, J. Power Sources

2017, 352, 9.[158] S. Yang, B. Yan, J. Wu, L. Lu, K. Zeng, ACS Appl. Mater. Interfaces

2017, 9, 13999.[159] W. Lu, L.-M. Wong, S. Wang, K. Zeng, J. Electrochem. Soc. 2016,

163, E147.[160] W. Lu, L.-M. Wong, S. Wang, K. Zeng, Phys. Chem. Chem. Phys.

2017, 19, 31399.[161] W. Lu, J. Xiao, L.-M. Wong, S. Wang, K. Zeng, ACS Appl. Mater.

Interfaces 2018, 10, 8092.

[162] W. Lu, L.-M. Wong, S. Wang, K. Zeng, J. Materiomics 2018, 4, 228.[163] A. Eshghinejad, E. N. Esfahani, P. Wang, S. Xie, T. C. Geary,

S. B. Adler, J. Li, J. Appl. Phys. 2016, 119, 205110.[164] E. Nasr Esfahani, A. Eshghinejad, Y. Ou, J. Zhao, S. Adler, J. Li,

Microsc. Today 2017, 25, 12.[165] J. Li, B. Huang, E. Nasr Esfahani, L. Wei, J. Yao, J. Zhao, W. Chen,

npj Quantum Mater. 2017, 2, 56.[166] E. Strelcov, Q. Dong, T. Li, J. Chae, Y. Shao, Y. Deng, A. Gruverman,

J. Huang, A. Centrone, Sci. Adv. 2017, 3, e1602165.[167] J. P. C. Fernandes, V. H. Mareau, L. Gonon, Int. J. Polym.

Anal. Charact. 2018, 23, 113.[168] W. Fu, W. Zhang, Small 2017, 13, 1603525.[169] R. Garcia, M. Chiesa, Y. K. Ryu, in Scanning Probe Microscopy for

Energy Research (Eds: D. A. Bonnell, S. V. Kalinin), World Scientific, Singapore 2013, p. 529.

[170] R. Garcia, N. S. Losilla, J. Martínez, R. V. Martinez, F. J. Palomares, Y. Huttel, M. Calvaresi, F. Zerbetto, Appl. Phys. Lett. 2010, 96, 143110.

[171] R. Garcia, R. V. Martinez, J. Martinez, Chem. Soc. Rev. 2006, 35, 29.[172] Z. Wei, D. Wang, S. Kim, S.-Y. Kim, Y. Hu, M. K. Yakes,

A. R. Laracuente, Z. Dai, S. R. Marder, C. Berger, W. P. King, W. A. de Heer, P. E. Sheehan, E. Riedo, Science 2010, 328, 1373.

[173] X. D. Dang, A. B. Tamayo, J. Seo, C. V. Hoven, B. Walker, T. Q. Nguyen, Adv. Funct. Mater. 2010, 20, 3314.

[174] S. Sadewasser, T. Glatzel, M. Rusu, A. Meeder, D. Fuertes Marrón, A. Jäger-Waldau, M. C. Lux-Steiner, MRS Proc. 2011, 668, H5.4.

[175] L. G. Joseph, N. M. Jeremy, Nanotechnology 2016, 27, 245705.[176] P. A. Fernández Garrillo, Ł. Borowik, F. Caffy, R. Demadrille,

B. Grévin, ACS Appl. Mater. Interfaces 2016, 8, 31460.[177] Z. Schumacher, A. Spielhofer, Y. Miyahara, P. Grutter,

Appl. Phys. Lett. 2017, 110, 053111.[178] J. Murawski, T. Graupner, P. Milde, R. Raupach,

U. Zerweck-Trogisch, L. M. Eng, J. Appl. Phys. 2015, 118, 154302.[179] D. C. Coffey, D. S. Ginger, Nat. Mater. 2006, 5, 735.[180] G. Shao, G. E. Rayermann, E. M. Smith, D. S. Ginger,

J. Phys. Chem. B 2013, 117, 4654.[181] R. Giridharagopal, G. E. Rayermann, G. Shao, D. T. Moore,

O. G. Reid, A. F. Tillack, D. J. Masiello, D. S. Ginger, Nano Lett. 2012, 12, 893.

[182] R. Proksch, J. Appl. Phys. 2014, 116, 066804.[183] R. Proksch, J. Appl. Phys. 2015, 118, 072011.[184] J. Yu, E. N. Esfahani, Q. Zhu, D. Shan, T. Jia, S. Xie, J. Li,

J. Appl. Phys. 2018, 123, 155104.[185] Q. N. Chen, Y. Ou, F. Ma, J. Li, Appl. Phys. Lett. 2014, 104, 242907.[186] N. Balke, P. Maksymovych, S. Jesse, A. Herklotz, A. Tselev,

C.-B. Eom, I. I. Kravchenko, P. Yu, S. V. Kalinin, ACS Nano 2015, 9, 6484.

[187] D. Seol, S. Park, O. V. Varenyk, S. Lee, H. N. Lee, A. N. Morozovska, Y. Kim, Sci. Rep. 2016, 6, 30579.

[188] D. Seol, B. Kim, Y. Kim, Curr. Appl. Phys. 2017, 17, 661.[189] T. Li, P. Sharma, A. Lipatov, H. Lee, J.-W. Lee, M. Y. Zhuravlev,

T. R. Paudel, Y. A. Genenko, C.-B. Eom, E. Y. Tsymbal, A. Sinitskii, A. Gruverman, Nano Lett. 2017, 17, 922.

[190] A. Lipatov, P. Sharma, A. Gruverman, A. Sinitskii, ACS Nano 2015, 9, 8089.

[191] X. Hong, A. Posadas, A. Lin, C. H. Ahn, Phys. Rev. B 2003, 68, 134415.

[192] H. S. Lee, S. W. Min, M. K. Park, Y. T. Lee, P. J. Jeon, J. H. Kim, S. Ryu, S. Im, Small 2012, 8, 3111.

[193] Z. Fan, J. Xiao, K. Sun, L. Chen, Y. Hu, J. Ouyang, K. P. Ong, K. Zeng, J. Wang, J. Phys. Chem. Lett. 2015, 6, 1155.

[194] Z. Fan, W. Ji, T. Li, J. Xiao, P. Yang, K. P. Ong, K. Zeng, K. Yao, J. Wang, Acta Mater. 2015, 88, 83.

[195] Q. Ke, A. Kumar, X. Lou, K. Zeng, J. Wang, J. Appl. Phys. 2011, 110, 124102.

Adv. Mater. 2018, 30, 1803064



[196] H. Liu, P. Yang, Z. Fan, A. Kumar, K. Yao, K. P. Ong, K. Zeng, J. Wang, Phys. Rev. B 2013, 87, 220101.

[197] W. Ong, C. Ke, P. Lim, A. Kumar, K. Zeng, G. W. Ho, Polymer 2013, 54, 5330.

[198] Q. Ke, A. Kumar, X. Lou, Y. P. Feng, K. Zeng, Y. Cai, J. Wang, Acta Mater. 2015, 82, 190.

[199] Y. Sun, Z. Hu, D. Zhao, K. Zeng, Nanoscale 2017, 9, 12163.[200] N. A. Hill, J. Phys. Chem. B 2000, 104, 6694.[201] J. Wang, J. B. Neaton, H. Zheng, V. Nagarajan, S. B. Ogale, B. Liu,

D. Viehland, V. Vaithyanathan, D. G. Schlom, U. V. Waghmare, N. A. Spaldin, K. M. Rabe, M. Wuttig, R. Ramesh, Science 2003, 299, 1719.

[202] T. Zhao, A. Scholl, F. Zavaliche, K. Lee, M. Barry, A. Doran, M. P. Cruz, Y. H. Chu, C. Ederer, N. A. Spaldin, R. R. Das, D. M. Kim, S. H. Baek, C. B. Eom, R. Ramesh, Nat. Mater. 2006, 5, 823.

[203] J. T. Heron, J. L. Bosse, Q. He, Y. Gao, M. Trassin, L. Ye, J. D. Clarkson, C. Wang, J. Liu, S. Salahuddin, D. C. Ralph, D. G. Schlom, J. Íñiguez, B. D. Huey, R. Ramesh, Nature 2014, 516, 370.

[204] Y.-H. Chu, L. W. Martin, M. B. Holcomb, M. Gajek, S.-J. Han, Q. He, N. Balke, C.-H. Yang, D. Lee, W. Hu, Q. Zhan, P.-L. Yang, A. Fraile-Rodríguez, A. Scholl, S. X. Wang, R. Ramesh, Nat. Mater. 2008, 7, 478.

[205] F. Zavaliche, H. Zheng, L. Mohaddes-Ardabili, S. Y. Yang, Q. Zhan, P. Shafer, E. Reilly, R. Chopdekar, Y. Jia, P. Wright, D. G. Schlom, Y. Suzuki, R. Ramesh, Nano Lett. 2005, 5, 1793.

[206] Q. N. Chen, F. Ma, S. Xie, Y. Liu, R. Proksch, J. Li, Nanoscale 2013, 5, 5747.

[207] J. Seidel, L. W. Martin, Q. He, Q. Zhan, Y. H. Chu, A. Rother, M. E. Hawkridge, P. Maksymovych, P. Yu, M. Gajek, N. Balke, S. V. Kalinin, S. Gemming, F. Wang, G. Catalan, J. F. Scott, N. A. Spaldin, J. Orenstein, R. Ramesh, Nat. Mater. 2009, 8, 229.

[208] Y. Geng, N. Lee, Y. J. Choi, S. W. Cheong, W. Wu, Nano Lett. 2012, 12, 6055.

[209] D. Meier, J. Phys.: Condens. Matter 2015, 27, 463003.[210] I. Vrejoiu, D. Preziosi, A. Morelli, E. Pippel, Appl. Phys. Lett. 2012,

100, 102903.[211] Z. Zhou, S. Zhao, Y. Gao, X. Wang, T. Nan, N. X. Sun, X. Yang,

M. Liu, Sci. Rep. 2016, 6, 20450.[212] L. W. Martin, Y.-H. Chu, Q. Zhan, R. Ramesh, S.-J. Han,

S. X. Wang, M. Warusawithana, D. G. Schlom, Appl. Phys. Lett. 2007, 91, 172513.

[213] M. Trassin, J. D. Clarkson, S. R. Bowden, J. Liu, J. T. Heron, R. J. Paull, E. Arenholz, D. T. Pierce, J. Unguris, Phys. Rev. B 2013, 87, 134426.

[214] W. Jiang, P. Upadhyaya, W. Zhang, G. Yu, M. B. Jungfleisch, F. Y. Fradin, J. E. Pearson, Y. Tserkovnyak, K. L. Wang, O. Heinonen, S. G. E. te Velthuis, A. Hoffmann, Science 2015, 349, 283.

[215] S. Seki, X. Z. Yu, S. Ishiwata, Y. Tokura, Science 2012, 336, 198.[216] J. S. White, I. Levatic, A. A. Omrani, N. Egetenmeyer, K. Prša,

I. Živkovic, J. L. Gavilano, J. Kohlbrecher, M. Bartkowiak, H. Berger, H. M. Rønnow, J. Phys.: Condens. Matter 2012, 24, 432201.

[217] P.-J. Hsu, A. Kubetzka, A. Finco, N. Romming, K. von Bergmann, R. Wiesendanger, Nat. Nanotechnol. 2016, 12, 123.

[218] N. Romming, A. Kubetzka, C. Hanneken, K. von Bergmann, R. Wiesendanger, Phys. Rev. Lett. 2015, 114, 177203.

[219] P. Milde, D. Köhler, J. Seidel, L. M. Eng, A. Bauer, A. Chacon, J. Kindervater, S. Mühlbauer, C. Pfleiderer, S. Buhrandt, C. Schütte, A. Rosch, Science 2013, 340, 1076.

[220] A. Soumyanarayanan, M. Raju, A. L. Gonzalez Oyarce, A. K. C. Tan, M.-Y. Im, A. P. Petrovic, P. Ho, K. H. Khoo, M. Tran, C. K. Gan, F. Ernult, C. Panagopoulos, Nat. Mater. 2017, 16, 898.

[221] Y. Heo, B. K. Jang, S. J. Kim, C. H. Yang, J. Seidel, Adv. Mater. 2014, 26, 7568.

[222] Y. Heo, S. Hu, P. Sharma, K.-E. Kim, B.-K. Jang, C. Cazorla, C.-H. Yang, J. Seidel, ACS Nano 2017, 11, 2805.

[223] P. Sharma, K. R. Kang, B. K. Jang, C. H. Yang, J. Seidel, Adv. Elec-tron. Mater. 2016, 2, 1600283.

[224] P. Sharma, Y. Heo, B. K. Jang, Y. Liu, V. Nagarajan, J. Li, C. H. Yang, J. Seidel, Adv. Mater. Interfaces 2016, 3, 1600033.

[225] P. Sharma, Y. Heo, B. K. Jang, Y. Y. Liu, J. Y. Li, C. H. Yang, J. Seidel, Sci. Rep. 2016, 6, 32347.

[226] P. Sharma, K. R. Kang, Y. Y. Liu, B. K. Jang, J. Y. Li, C. H. Yang, J. Seidel, Nanotechnology 2018, 29, 205703.

[227] A. Alsubaie, P. Sharma, G. Liu, V. Nagarajan, J. Seidel, Nanotech-nology 2017, 28, 075709.

[228] Ü. Özgür, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S.-J. Cho, H. Morkoç, J. Appl. Phys. 2005, 98, 041301.

[229] T. S. Herng, M. F. Wong, D. Qi, J. Yi, A. Kumar, A. Huang, F. C. Kartawidjaja, S. Smadici, P. Abbamonte, C. Sánchez-Hanke, S. Shannigrahi, J. M. Xue, J. Wang, Y. P. Feng, A. Rusydi, K. Zeng, J. Ding, Adv. Mater. 2011, 23, 1635.

[230] W. L. Ong, H. Huang, J. Xiao, K. Zeng, G. W. Ho, Nanoscale 2014, 6, 1680.

[231] Y. Kim, A. Kumar, O. Ovchinnikov, S. Jesse, H. Han, D. Pantel, I. Vrejoiu, W. Lee, D. Hesse, M. Alexe, S. V. Kalinin, ACS Nano 2012, 6, 491.

[232] J. Zhu, J. Feng, L. Lu, K. Zeng, J. Power Sources 2012, 197, 224.[233] J. Zhu, K. Zeng, L. Lu, J. Appl. Phys. 2012, 111, 063723.[234] A. Clémençon, A. T. Appapillai, S. Kumar, Y. Shao-Horn, Electro-

chim. Acta 2007, 52, 4572.[235] N. Balke, S. Kalnaus, N. J. Dudney, C. Daniel, S. Jesse, S. V. Kalinin,

Nano Lett. 2012, 12, 3399.[236] S. Jesse, A. Kumar, T. M. Arruda, Y. Kim, S. V. Kalinin, F. Ciucci,

MRS Bull. 2012, 37, 651.[237] S. Yang, B. Yan, T. Li, J. Zhu, L. Lu, K. Zeng, Phys. Chem.

Chem. Phys. 2015, 17, 22235.[238] S. Yang, B. Yan, L. Lu, K. Zeng, RSC Adv. 2016, 6, 94000.[239] K. Zeng, T. Li, T. Tian, J. Phys. D: Appl. Phys. 2017, 50, 313001.[240] C. Zhao, Y. Lu, Y. Li, L. Jiang, X. Rong, Y.-S. Hu, H. Li, L. Chen,

Small Methods 2017, 1, 1600063.[241] B. Ji, F. Zhang, N. Wu, Y. Tang, Adv. Energy Mater. 2017, 7,

1700920.[242] J. S. Yun, A. Ho-Baillie, S. Huang, S. H. Woo, Y. Heo, J. Seidel,

F. Huang, Y.-B. Cheng, M. A. Green, J. Phys. Chem. Lett. 2015, 6, 875.[243] J. S. Yun, J. Seidel, J. Kim, A. M. Soufiani, S. Huang, J. Lau,

N. J. Jeon, S. I. Seok, M. A. Green, A. Ho-Baillie, Adv. Energy Mater. 2016, 6, 1600330.

[244] Y. C. Kim, N. J. Jeon, J. H. Noh, W. S. Yang, J. Seo, J. S. Yun, A. Ho-Baillie, S. Huang, M. A. Green, J. Seidel, T. K. Ahn, S. I. Seok, Adv. Energy Mater. 2016, 6, 1502104.

[245] N. Faraji, C. Qin, T. Matsushima, C. Adachi, J. Seidel, J. Phys. Chem. C 2018, 122, 4817.

[246] J. Kim, J. S. Yun, X. Wen, A. M. Soufiani, C. F. J. Lau, B. Wilkinson, J. Seidel, M. A. Green, S. Huang, A. W. Y. Ho-Baillie, J. Phys. Chem. C 2016, 120, 11262.

[247] M. Zhang, J. S. Yun, Q. Ma, J. Zheng, C. F. J. Lau, X. Deng, J. Kim, D. Kim, J. Seidel, M. A. Green, S. Huang, A. W. Y. Ho-Baillie, ACS Energy Lett. 2017, 2, 438.

[248] D. S. Lee, J. S. Yun, J. Kim, A. M. Soufiani, S. Chen, Y. Cho, X. Deng, J. Seidel, S. Lim, S. Huang, A. W. Y. Ho-Baillie, ACS Energy Lett. 2018, 3, 647.

[249] Y. Cho, A. M. Soufiani, J. S. Yun, J. Kim, D. S. Lee, J. Seidel, X. Deng, M. A. Green, S. Huang, A. W. Y. Ho-Baillie, Adv. Energy Mater. 2018, 8, 1703392.

[250] J. S. Yun, J. Kim, T. Young, R. J. Patterson, D. Kim, J. Seidel, S. Lim, M. A. Green, S. Huang, A. Ho-Baillie, Adv. Funct. Mater. 2018, 28, 1705363.

[251] X. Liu, Y. Zhang, L. Shi, Z. Liu, J. Huang, J. S. Yun, Y. Zeng, A. Pu, K. Sun, Z. Hameiri, J. A. Stride, J. Seidel, M. A. Green, X. Hao, Adv. Energy Mater. 2018, 1800138.

Adv. Mater. 2018, 30, 1803064



[252] F. Nastaran, U. Clemens, W. Niklas, K. Lorenz, A. Rainer, M. Y. Kumar, S. Jan, Adv. Electron. Mater. 2016, 2, 1600138.

[253] J. M. Frost, K. T. Butler, A. Walsh, Apl. Mater. 2014, 2, 081506.[254] A. Stroppa, D. Di Sante, P. Barone, M. Bokdam, G. Kresse,

C. Franchini, M.-H. Whangbo, S. Picozzi, Nat. Commun. 2014, 5, 5900.

[255] S. Liu, F. Zheng, N. Z. Koocher, H. Takenaka, F. Wang, A. M. Rappe, J. Phys. Chem. Lett. 2015, 6, 693.

[256] I. M. Hermes, S. A. Bretschneider, V. W. Bergmann, D. Li, A. Klasen, J. Mars, W. Tremel, F. Laquai, H.-J. Butt, M. Mezger, R. Berger, B. J. Rodriguez, S. A. L. Weber, J. Phys. Chem. C 2016, 120, 5724.

[257] M. Szafranski, Cryst. Growth Des. 2016, 16, 3771.[258] Y. Li, M. Behtash, J. Wong, K. Yang, J. Phys. Chem. C 2018, 122,

177.[259] J. Zhao, Y. Deng, H. Wei, X. Zheng, Z. Yu, Y. Shao, J. E. Shield,

J. Huang, Sci. Adv. 2017, 3, eaao5616.[260] H. Rohm, T. Leonhard, M. J. Hoffmann, A. Colsmann, Energy

Environ. Sci. 2017, 10, 950.[261] D. Rossi, A. Pecchia, M. A. d. Maur, T. Leonhard, H. Röhm,

M. J. Hoffmann, A. Colsmann, A. D. Carlo, Nano Energy 2018, 48, 20.[262] P. Wang, J. Zhao, L. Wei, Q. Zhu, S. Xie, J. Liu, X. Meng, J. Li,

Nanoscale 2017, 9, 3806.[263] B. Huang, G. Kong, E. N. Esfahani, S. Chen, Q. Li, T. Yu, N. Xu,

Y. Zhang, S. Xie, H. Wen, P. Gao, J. Zhao, J. Li, npj Quantum Mater. 2018, 3, 30.

[264] T. Li, K. Zeng, Acta Mater. 2011, 59, 3667.[265] T. Li, K. Zeng, J. Struct. Biol. 2012, 180, 73.

[266] T. Li, L. Chen, K. Zeng, Acta Biomater. 2013, 9, 5903.[267] F. Barthelat, J. E. Rim, H. D. Espinosa, in Applied Scanning Probe

Methods XIII: Biomimetics and Industrial Applications, Vol. XIII (Eds: B. Bhushan, H. Fuchs), Springer, New York 2009, p. 17.

[268] P. Yu, C. Ning, Y. Zhang, G. Tan, Z. Lin, S. Liu, X. Wang, H. Yang, K. Li, X. Yi, Y. Zhu, C. Mao, Theranostics 2017, 7, 3387.

[269] A. Gruverman, B. J. Rodriguez, S. V. Kalinin, in Scanning Probe Microscopy, Vol. III (Eds: S. V. Kalinin, A. Gruverman), Springer, New York 2007, p. 615.

[270] P. H. Byers, S. M. Pyott, Annu. Rev. Genet. 2012, 46, 475.[271] Z. Yanhang, L. Jiangyu, S. B. Gregory, J. Phys. D: Appl. Phys. 2017,

50, 133001.[272] S. M. Damaraju, Y. Shen, E. Elele, B. Khusid, A. Eshghinejad, J. Li,

M. Jaffe, T. L. Arinzeh, Biomaterials 2017, 149, 51.[273] D.-W. Fu, H.-L. Cai, Y. Liu, Q. Ye, W. Zhang, Y. Zhang, X.-Y. Chen,

G. Giovannetti, M. Capone, J. Li, R.-G. Xiong, Science 2013, 339, 425.

[274] S. Horiuchi, Y. Tokunaga, G. Giovannetti, S. Picozzi, H. Itoh, R. Shimano, R. Kumai, Y. Tokura, Nature 2010, 463, 789.

[275] H. Lu, T. Li, S. Poddar, O. Goit, A. Lipatov, A. Sinitskii, S. Ducharme, A. Gruverman, Adv. Mater. 2015, 27, 7832.

[276] J. Li, Y. Liu, Y. Zhang, H.-L. Cai, R.-G. Xiong, Phys. Chem. Chem. Phys. 2013, 15, 20786.

[277] Z. Hu, Y. Sun, K. Zeng, D. Zhao, Chem. Commun. 2017, 53, 8653.[278] G. Shao, M. S. Glaz, F. Ma, H. Ju, D. S. Ginger, ACS Nano 2014,

8, 10799.[279] M. Takihara, T. Takahashi, T. Ujihara, Appl. Phys. Lett. 2008, 93,

021902.

Adv. Mater. 2018, 30, 1803064

Documents

Probing of Local Multifield Coupling Phenomena of Advanced ...csqs.xjtu.edu.cn/17.pdf · Probing of Local Multifield Coupling Phenomena of Advanced Materials by Scanning Probe Microscopy