The effect of crystal structure on the electromechanical properties of piezoelectric Nylon-11 nanowires

Mechanical and piezoelectric properties of Nylon-11 nanowires strongly depend on their crystal structure, which can be precisely controlled by template-assisted growth methods.


Methods
Materials The Nylon-11 solution was prepared by mixing Nylon-11 pellets (Sigma-Aldrich, Mw = 201.31 g mol -1 ) and formic acid (Sigma-Aldrich, Reagent grade > 95%) on a hot plate at 90°C. An Anodisc AAO (anodic aluminium oxide, Whatman) 25mm template was used to form the nanowires, with a pore width of 200 nm.
Fabrication of nanowires by conventional template wetting The AAO template was placed on top of the 10 wt% Nylon-11 solution. This assembly was then left at room temperature with no additional gas flow.
Fabrication of α-phase nanowires To make α-phase nanowires, 5 wt% Nylon-11 solution was dropped onto a cleaned petri as described in the conventional template wetting method. However, in this method the AAO template was attached to a square glass slide to limit the exposure of the top surface of the AAO template to the air, thus limiting the rate of formic acid evaporation. A lid was then placed over the petri dish to further reduce exposure to the surrounding air, and the dish was then placed on a hot plate which was pre-heated to 40°C.
Fabrication of δʹ-phase nanowires To make δʹ-phase nanowires, a bespoke fabrication process 5 was utilised. An AAO template was placed on top of a drop of 17.5wt% Nylon-11 solution in accordance with the conventional template wetting method. No additional protective layers were added, and the solution was not heated. This dish was then placed in a fume cupboard, and a desktop fan was positioned next to the assembly such that air was blown over the surface of the template. The fan was set to blow air at a speed of ~ 3 m s -1 , and an anemometer was used to record the flow velocity.
Fabrication of α-phase Nylon-11 films The α-phase Nylon-11 films were produced by drop-casting a 10 wt% solution on the hot plate (40°C).
Preparation of template-freed nanowires This was achieved by first pouring a 40 vol% phosphoric acid solution over the sample and left for 3-4 hours. The resulting nanowire mat was then washed in distilled water and left to air-dry.

Characterization
The field-emission scanning electron microscopy (FE-SEM, FEI Nova Nano SEM) was used to investigate the morphology of the nanowires. The crystal structure of Nylon-11 nanowires with and without the AAO template was observed by an X-ray diffraction (XRD) machine (Bruker D8) using Cu Ka radiation (l = 1.5418 A) with a silicon substrate. QNM and PFM measurements were carried out using Bruker Multimode 8 with Antimony (n) doped Si (tip radius ~ 35 nm, resonance frequency 150 kHz). AC voltages were applied from a lock-in amplifier. To measure the piezoelectric response, one side of the Nylon-11 NW filled AAO template and the α-phase film was coated by sputter (using k550 Emitech). The α-phase film produces two distinct peaks at 2θ = 20.02° and 23.01°, corresponding to the (200) and (210/010) planes respectively. There is also a small peak at 7.5°, corresponding to the (001) plane. Nanowires without AAO sample shows all possible diffraction peaks because nanowires are randomly oriented in the sample. In contrast, only family of peaks in the diffraction patterns are observed from the nanowires within AAO. (Fig. S4a)   Fig. S4b depicts the x-ray diffraction with different lattice directions. In the case of (200) plane, a Bragg-Brentano diffractometer produce a diffraction peak at corresponding 2θ angle as Bragg's law is fulfilled. The (210) plane would diffract, however, only background peak is observed because the lattice planes are not aligned with scattering vector (q).   QNM calibration was performed according to the manufacturer's (Bruker) instructions. First of all, the tip calibration (including deflection sensitivity measurement and thermal tune) was conducted using standard sapphire samples. Then, to calibrate the measuring condition, the DMT modulus of a Polystyrene (PS) film (Bruker) was recorded. Since the DMT value of a standard PS film is already known (2 ~ 3 GPa), we tuned parameters such as peak force amplitude and frequency