High-sensitivity piezoresponse force microscopy studies of single polyvinylidene fluoride nanofibers
Graphical abstract
Introduction
Piezoresponse force microscopy (PFM) opens up a novel perspective on exploring the piezoelectric properties of the piezoelectric materials at nanoscale level [1]. Its popularity is due to the fact that the PFM provides direct experimental evidences on the interplay between the domain switching kinetics and microstructural features. Particularly, the PFM allows for the well-justified interpretation of the electromechanical coupling underlying many micro-/nanostructures [2], yet it is still a challenging issue when the PFM technique is applied to single electrospun nanofibers. First, it is difficult to fix the individual nanofiber on a conductive substrate. Second, the resolution is limited by the size of a PFM tip if the fiber diameter is less than 50 nm because of the thermal drift of the PFM system. In a typical PFM measurement, as we know, the tip applied with a voltage bias is brought into contact with the surface of a sample, and the tip deflection is induced from the expansion and contraction of the sample. The amplitude of the out-of-plane (OP) reflection is defined as the piezoelectric coefficient (d33) of the material, and the phase describes its piezoelectric polarity. This method is suggested as an ideal tool for probing local piezoelectric properties. In fact, no other techniques are capable of probing directly the spontaneous polarization switching at nanoscale level [3].
Polyvinylidene fluoride (PVDF) and its copolymers exhibit piezoelectric, ferroelectric, pyroelectric, and electro-cooling effects. Moreover, owing to its polymeric nature, PVDF is more flexible, and lightweight and processable than conventional piezoelectric ceramics. Therefore, the piezoelectricity of the polymeric materials has attracted considerable interest in their applications in implantable biosensors and wearable and portable electronic devices [4], [5]. Recently, the piezoelectric response imaging of some polymeric films or fibers was realized, but the further quantitative analysis of the thinner nanofibers such as piezoelectric switching behavior and d33 is seldom [2].
In this work, a series of PVDF nanofibers were prepared on the Pt/Ti/SiO2/Si substrate by a far-field electrospinning technique. PFM was employed to study spatial imaging of piezoelectric switching and local piezoelectric response in the single nanofibers. Their diameter-dependent piezoelectric property was also studies. In addition to confirming the origin of their OP piezoelectricity, the PFM results reveal the details of stepwise amplitude and phase changes during the periodic polarization switching.
Section snippets
Experimental details
Electrospinning was performed on a SiO2/Si substrate (bottom electrode) previously sputter coated with a Ti bottom layer and then a Pt top layer because the Pt has high surface energy. 16 wt% PVDF (Mw = ∼534,000 g mol−1, Sigma–Aldrich, USA) was dissolved in a solvent (4:6 DMAC and acetone) under stirring for 2 h at 60 °C [6]. The polymer solution was loaded into a plastic syringe (50 mL) fitted with a stainless steel needle (25G). A DC voltage of 30 kV was applied to the needle from a high voltage
Results and discussion
With the AC bias applied between the PFM tip and the bottom electrode, the PFM allows for recording of the vertical piezoelectric response (VPR) and the lateral piezoelectric response (LPR) using the lock-in techniques [1]. The VPR reflects the cantilever vibrations in the vertical direction, while the LPR is described by the cantilever torsion in the length direction. Fig. 2 shows topography (a, b), VPR (c, d), and LPR (e, f) images (the so-called efficient piezoresponse signal) measured at
Conclusions
In summary, switching spectroscopy dual AC resonance tracking technique was adopted to quantitatively capture 2D piezoelectric response of electrospun PVDF nanofibers with high sensitivity. The PVDF nanofibers have much stronger out-of-plane piezoelectric response signal, which is difficult to probe by conventional techniques. On the other hand, this technique allows to detect the local polarization switching in a controllable period, which can determine the piezoelectric coefficient (d33)
Acknowledgments
This work was supported by the Grants from the National Natural Science Foundation of China (Nos. 61474071, 61531166006) and National Basic Research Program (973 Program, No. 2015CB352106).
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