Novices will learn the techniques directly from the inventors and senior researchers will gain in-depth information on the new technologies that are suitable for advanced analysis. On the one hand, fundamental concepts that are needed to understand the nanomechanical behavior of materials is included in the introductory part of the book. On the other hand, dedicated chapters describe the utilization of advanced numerical modeling in understanding the properties of complex materials.
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Handbook of Thin Film Technology. Atmospheric Pressure Plasma Treatment of Polymers. Sintering of Advanced Materials. Probability Based High Temperature Engineering. Analysis of force curves is performed on the fly as the tip is scanned across the sample, resulting in the acquisition of spatially resolved mechanical properties. This imaging mode is non-destructive to both tip and sample since it directly controls the peak normal force and minimizes the lateral force on the probe. Lateral in-plane deflections were observed only from SA-CNFs oriented parallel to the AFM cantilever, which demonstrated the presence of a selective shear piezoelectric response, and could be correlated to the hierarchical structural features in these SA-CNFs as separately revealed by scanning electron microscopy SEM and high resolution transmission electron microscopy TEM.
Noticeably, the SA-CNFs were found to remain attached to the residual cellulose film from the template-wetting process schematically shown in Fig. The observed surface roughness may have arisen due to the rod-like CNCs structures protruding from the surface see schematic in Fig. Low-temperature post-deposition thermal treatment process was found to be necessary to realise SA-CNFs of higher crystallinity and improved mechanical stability, which are essential to obtain better piezoelectric response in cellulose. A schematic of the self-assembled structure is presented in Fig.
High resolution imaging Fig. In our case, we suggest that the confinement of the CNCs within the nanopore channels forces the self-assembly process to rod-like clusters, and finally to SA-CNFs, as driven by the surface energy of the template pores, which results in the morphological integrity being retained even after freeing from the template. Although the exact self-assembly mechanism is not yet clear, it might be related to the attraction due to the charged AAO template walls We suggest that the interaction between the template walls and the nanorods determines the initial configuration nucleation of the SA-CNFs, which is subsequently replicated to form the rest of the structure.
A possible route is as follows: When considering the nucleation of fibre growth on the curved wall, a chiral rod, having a twisted facet, will have a preferential alignment in an off-axis orientation compared to the nanopore axis. This alignment will conserve the right-handed chirality, unlike rod-to-rod interactions resulting in chirality inversion see ESI in ref. This off-axis, right-handed, chiral nucleation may override other effects which dominate self-assembly in the absence of a template.
When an electric field is applied across the sample via an AFM tip, a piezoelectric response is observed in the form of mechanical deformation of the sample at the point of contact. For any given deformation, there are two in-plane orientations, and one out-of-plane. The AFM cantilever picks up the out-of-plane deformations as a vertical deflection signal. The in-plane deformation in the direction transverse to the cantilever axis results in a lateral deflection signal, while the in-plane deformation in the direction parallel to the cantilever may give rise to buckling in the cantilever, thus producing a vertical deflection signal as well.
Considering there is a background noise value even when measuring a non-piezoelectric conductive sample e. The difference of about 1—2 pm V —1 between SA-CNF values and the ITO, corresponds well with wood piezoelectricity, 13 however it is smaller than reports for processed CNC films, which are 5—10 times larger, 32 , 33 attributed to mechanical pressure and pre-poling, as mentioned above. Piezoelectric optimization was not performed in this study, and we focus our discussion on the directionality of arising piezoelectric signals in light of the self-assembly process, and the nanoscale measurements.
The vertical measurement is typically considered to arise from a d ii type of piezo-response, i. Nonetheless, in-plane deformations in the direction parallel to the cantilever, will also result in a vertical signal, due to buckling mode excitation, 56 as explained above. The dashed circles in Fig. When considering lateral cantilever displacements, it is important to note that only in-plane deformations transverse to the cantilever will result in a measured lateral signal.
Interestingly, we found that there was no considerable signal comparing the signals from the SA-CNF and the substrate when the SA-CNF and cantilever were in the transverse configuration scan 1. This signal is much smaller than the measured vertical signal, however the calibration of the lateral signal is not straightforward and was based here on a geometrical calculation.
As discussed earlier, the preferential orientation is possible due to the porous template-assisted formation, as has been demonstrated for other materials. Earlier measurements performed without surface potential minimization, have shown similar trends however are less reliable quantitatively; see ESI Fig. This schematic corresponds to the TEM observation Fig.
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The rods on the bottom part bottom solid arrow , forming a helical mirror image, will however form an opposite reaction in the rod coordinate system, which may result in the cancellation of the vertical and axial deformation and enhancement of the transversal one, due to the helical arrangement. Furthermore, the tip-CNF geometry induces inhomogeneous electric field in the material which may give rise to further complexity in the piezoelectric deformation.
This would correspond to flattening the wire depicted in Fig. Two types of piezoelectric coupling matrices were considered: Since the bottom contact is treated as mechanically clamping the structure, a thickness asymmetry was introduced to intensify the rotated bilayer effect, such that the top and bottom layer thicknesses are 5 and 25 nm, correspondingly. It is important to note that it is not straightforward to tie the simulation results with PFM signals.
For example, the vertical deformation y -axis in this case , although distinctive, will probably not result in actual vertical PFM signal. This is due to the fact that the deformation has a different sign on opposite sides of the tip, and it is more likely to result in an effectively measured lateral signal or buckling signal, which may be disguised as a vertical signal depending on tip orientation.
The simulation results raise several interesting points: While this is to be expected, it bears consequences for previously reported results, where d 33 type excitations were interpreted as d 25 values. Indeed, in light of this, a paradigm shift is required when analysing piezoelectricity of highly oriented cellulose films.
Our simulations indicate that the non-degenerate matrix is better suited to model the experimental results. The main result of the simulation is that the bilayer structure acts to prevent the deformation pattern to follow that of a single layer, and directs the response for certain axes: The z -axis deformation middle row is relatively maintained, although in an opposite trend to the vertically-generated lateral signal.
The x -axis deformation is diminished compared to the single layer case. Furthermore, when considering only one layer, the rotation of the layer basically causes the transfer of z -axis deformation to x -axis. This is marked by the solid and dashed squares. Future work will take into account the internal structure of the SA-CNFs, which probably induces additional degrees of freedom, compared to a single domain as examined here.
ND-PFM measurements on individual annealed SA-CNFs were found to have a predominant lateral piezo-response in the axial direction, which we attribute to their chirality. Finite element simulations were found to corroborate this explanation. Given that cellulose is already an attractive material for biodegradable and wearable sensors, 9 , 25 , 28 — 31 , 33 , 34 the facile fabrication approach for SA-CNFs presented here, as well as the multi-microscopy approach adopted to understand their fundamental structure and piezoelectricity, may pave the way for studying self-assembly in other piezoelectric chiral phase biomaterials.
Our studies therefore offer insight into possible routes towards engineering nanofibers with tailor-made electromechanical properties, by controlling the way in which chiral-nematic liquid crystals self-assemble via template-assisted nano-confinement. Then, the sample was diluted 10 times in cold double distilled water DDW , and the mixture was left standing for 1 hour.