Journal Club Theme of May 2011: Nanoscale Electromechanics and Piezoresponse Force Microscopy

Coupling between electrical and mechanical phenomena is ubiquitous in nature and underpins the functionality of materials and systems as diversified as ferroelectrics and multiferroics, electroactive molecules, and biological systems. In ferroelectrics, electromechanical behavior is directly linked to polarization order parameter and hence can be used to study complex phenomena including polarization reversal, domain wall pinning, multiferroic interaction, and electron-lattice coupling. The very basis of functionalities of biological systems is electromechanics - from nerve-controlled muscle contraction on macroscale to cardiac activity and hearing on microscale and to energy storage in mitochondria, voltage-controlled ion channels and electromotor proteins on nanoscale. More broadly, electromechanical coupling is a key component of virtually all electrochemical transformations, and is a nearly universal part of energy conversion and transport processes. It forms a basis for many device applications, and is directly relevant to virtually all existing and emerging aspects of materials science and nanobiotechnology.

The ubiquity and importance of electromechanics is belied by the lack of systematic interdisciplinary studies, due to, until recently, the dearth of corresponding nanoscale probing tools and the difficulty in quantitatively determining the relatively small electromechanical coupling coefficients. The development of piezoresponse force microscopy (PFM) in the last decade has led to rapid advances in the investigation of electromechanics with unprecedented resolution. In ferroelectric materials, PFM has enabled imaging of static and dynamic domain characteristics at the nanometer level [1], providing direct experimental observations on switching and fatigue [2,3], domain-defect interactions [4], and nucleation mechanisms [5]. The last several years have also witnessed a number of spectacular advances in PFM imaging and characterization of III-V nitrides [6], ferroelectric and biological polymers [7], and expanding PFM capabilities to liquid and vacuum environments [8], and some excellent reviews can be found in [1,9,10]. Furthermore, a series of international workshops have been held in the past a few years, including a recent one in Beijing last summer. The presentations of that workshop can be found at http://mse.ustb.edu.cn/files/pfm/Program.html. Here we briefly review the principle of PFM and its applications to ferroelectrics and multiferroics, biological systems, and electrochemical systems. The opportunities and challenges it provides to mechanics community will also be discussed.

1.    Principles of Piezoresponse Force Microscopy

The very first paper on PFM was published in 1992 by Guthner and Dransfeld in Applied Physics Letters, though the term piezoresponse force microscopy was coined three years later by Alexei Gruverman. The principle of PFM is straightforward conceptually – a conductive scanning probe microscopy (SPM) tip is used to apply an AC voltage to the specimen and induce a local displacement that is recorded by photodiode if the specimen is piezoelectric. The magnitude of such displacement is proportional to the piezoelectric coefficient, while the phase is correlated with the polarity of the polarization, making it possible to image the domain structure of ferroelectrics. However, the implementation and analysis is not trivial, since the piezoelectric displacement is usually very small, in the order of picometers. To enhance the sensitivity, resonance enhancement by driving the AC voltage near the resonance of the cantilever-specimen system is often used [11] at the expense of quantitative analysis of piezoelectric coefficient. To overcome this difficulty, dual frequency resonance tracking (DFRT) [12] and band excitation (BE) [13] techniques have been developed, making it possible to quantitatively determine the phase, amplitude, quality factor, and resonance frequency simultaneously, and thus enable the quantitative analysis of local piezoelectric response of the sample. While most of the PFM experiments use vertical mode based on piezoelectric coefficient d33 to map the out-of-plane polarization distribution [14], lateral PFM based on piezoelectric coefficient d15 has also been developed to map the in-plane polarization distribution [15,16], and three-dimensional PFM has also been attempted by combining vertical and lateral PFM together [17]. In addition to mapping the domain structure of ferroelectrics, it is also possible to manipulate the domain structure using PFM, by imposing a sequence of DC voltage on top of AC voltage while measuring the piezoresponse simultaneously, resulting in local domain switching and hysteresis and butterfly loops [18]. The PFM hysteresis loop, however, measures piezoresponse phase versus DC voltage, and does not really give conventional hysteresis of polarization versus electric field. If that is desirable, the SPM tip should be connected to ferroelectric analyzer. Switching spectroscopy techniques has also been developed to map the local switching characteristics [19].

2.    Applications of Piezoresponse Force Microscopy

Piezoresponse force microscopy has been applied to study a wide range of ferroelectric materials in the past decades, and here we can only give a few snapshots of its applications. These include BaTiO3 [20], LiNbO3 [21], and SBN [22] crystals, PZT thin films [23], PMN-PT relaxors [24], P(VDF-TrFE) polymers [25], and low-dimensional ferroelectrics, such as nanowires [26] and nanofibers [7]. In the past a few years, PFM has played a major role in understanding magnetoelectric coupling in multiferroics at nanoscale, particularly in BiFeO3, that has stimulated great excitement in the field [27]. It has also been used as nanolithography tool to write a pre-designed domain structure by applying a distribution of positive and negative voltages based on pre-designed templates while scanning the sample [28]. Combining such domain structure with photovoltaic effect of ferroelectrics, photovoltaic assisted deposition of metallic pattern on ferroelectric surface has also demonstrated [29], taking advantages of directionality of photovoltaic current guided by the polarization of ferroelectric domains.

Another emerging application of PFM, which is still in its early development, is in biological systems. In 1960s, piezoelectric effects have been observed in a number of soft and hard tissues [30,31], which is believed to be a universal property of living tissues that may play a significant role in several physiological phenomena [32], for example the remodeling of bones and the formation of thrombi due to injury of blood vessels. To understand these phenomena, local probing of the piezoelectricity at nanoscale is essential, and PFM offers such powerful capability, especially with the recent advances of PFM in liquid environment [8]. Nevertheless, the applications of PFM in biological systems are still relatively limited, and a number of systems have been investigated by groups of Rodriguez, Gruverman, and Yu, among others. Some excellent reviews can be found in [10].

Recently, another important application of PFM has emerged, by demonstrating its applicability in Li-ion batteries, which is referred as electrochemical strain microscopy (ESM) [33]. It is well known that during Li-ion intercalation into and extraction from the electrodes, large lattice distortion is induced, which has substantial influence on the reliability and cyclic stability on the battery. It is highly desirable to probe such coupling between electrochemical process and deformation locally at nanoscale, and ESM, which is essentially identical to PFM, has recently been developed to map the deformation field in the electrodes due to Li-ion redistribution induced by external electric field [34]. Given the promises of nanostructured electrodes in high performance electric energy storage, we expect ESM will play a prominent role in the future development of Li-ion batteries.

3.    Opportunities and Challenges to Mechanics Community

The principles of PFM and ESM are based on nanoscale electromechanics, the intimately coupling between electric process and deformation. In return, they also offer new tools and opportunities to mechanics community, and enable potential investigations that would be difficult to carry out otherwise. One example would be fracture in ferroelectrics, for which the role of domain switching near crack tip remain to be hotly debated. Piezoresponse force microscopy now offers the capability to image such domain switching associated with crack propagation, and thus may help answer the question. As mentioned earlier, it also offers insight in understanding the fracture in Li-ion battery electrodes. Enormous opportunities also exist in biomechanics, where PFM can help clarifying electromechanical processes that seems ubiquitous in biological systems.

Piezoresponse force microscopy and electrochemical strain microscopy also impose tremendous challenges to mechanics community. From analysis point of view, the electric field underneath a SPM tip is highly concentrated, and the deformation induced by SPM tip is also local, constrained by the neighboring environment. The material’s heterogeneity and nonlinearity also have substantial influence on the piezoresponse, and the dynamics of cantilever and contact make the problem even worse.  All of these make the interpretation of data, especially quantitative analysis, extremely difficult, and these are challenges that mechanics community can make major contributions in helping overcome.

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