In-Situ Electromechanical Testing of ZnO Nanowires

Nanotubes and nanowires are one-dimensional structures that are potential materials for future optoelectronics, nanoelectronics, sensors, actuators and piezoelectric devices.

Since nanostructures have higher surface-to-volume ratios and length scale effects, they can have excellent electrical and mechanical properties, as well as other properties that are dependent on length scale. To utilize these key advantages, the unique characteristics of the nanostructures should be investigated and understood as a function of the material parameters. Despite the great technological advancements made over the last 10 years, complete electromechanical characterization of an individual nanowire is still considered to be a challenging task.

Electrical Push-to-Pull (E-PTP)

For electromechanical characterization of materials, a MEMS-based uniaxial nanotensile testing device with built-in four-probe electrical contacts was designed, as illustrated in Figures 1–4. Together with Bruker’s Hysitron^® PI Series PicoIndenter^® instruments, the small size of the E-PTP device, (2.5 mm x 2 mm x 0.4 mm) enables direct observation of the deformation process in SEM and TEM.

Figure 1. E-PTP mounted on the Hysitron PI 85L. Four screws (top) and gold leads (bottom) marked in red indicate the electrical connections for four probes.

Figure 2. Low-magnification SEM image displaying where the probe contacts the E-PTP (top) and high-magnification image showing sample mounting position (bottom).

E-PTP Features and Benefits

E-PTP devices provide several benefits for simultaneous in-situ mechanical and electrical measurements:

• E-PTP devices allow tensile testing of individual nanowires with high resolution of the displacement and force, simultaneously measuring electrical properties by the four-point probe method.
• During sample mounting, a nanowire is picked and placed with a nanomanipulator on the flexure and this nanowire is subsequently welded across the 2.5 µm gap.
• Separation of voltage-sensing and current-sourcing electrodes eliminates contact and lead resistance on the measurement, thus enabling accurate measurements of the electrical properties.
• Through uniaxial tensile testing with the E-PTP, substrate effects are prevented and a uniform deformation field is applied along the length of the sample.
• During the mechanical test, I-V curves are produced that help in the correlation of electrical to mechanical transitions in a material.
• In combination with Hysitron PI Series PicoIndenter instruments, E-PTP devices help in understanding the microscopic origin of electrical property changes in the sample under loading.

ZnO Nanowire on E-PTP

Non-centrosymmetric wurtzite structure zinc oxide (ZnO) exhibits a strong piezoelectric tensor among tetrahedrally bonded semiconductors, and is a potential material for actuators, sensors and energy harvesting applications. To understand the piezoelectric response of ZnO as a function of strain, uniaxial tensile tests with a four-point probe electrical measurement were carried out through E-PTP. Nanowires measuring ~20 micron length and ~300 nm diameter were placed on a E-PTP inside a dual-beam SEM-FIB. Next, Pt-based gas injection system (GIS) was employed for welding and for connecting four leads to the sample, as shown in Figure 3 (top).

In-Situ E-PTP Experiments

Strain-rate controlled tensile experiments were carried out inside an FE SEM using the Hysitron PI 85L SEM PicoIndenter in Electrical Characterization Module (ECM) mode. Then, using a Keithley 2602A SourceMeter, current was sourced and voltage was measured in four-probe mode. I-V curves were obtained by performing voltage sweeps at constant strain so as to extract electrical properties.

Periodic variation of the applied current or voltage generated periodic strain or stress from the nanowires. In-situ study provides a major advantage — i.e., diameter and length of the nanowire can be measured instantly from video frames and correlated with mechanical response. Using force-displacement data, true strain and true stress were determined and plotted as shown in Figure 3 (bottom).

The actual force to the nanowire (P_sample) was established by subtracting the force measured using the spring stiffness of the E-PTP (P_E-PTP) from the experimentally quantified load (P), that is, P_sample = P–P_E-PTP. Following the fracture of the nanowire, the stiffness of this E-PTP device was measured and was found to be 320 N/m.

Figure 3. ZnO nanowire mounted on E-PTP and connected to four leads (top). Stress–strain and resistance–strain curves (bottom).

Piezoelectric Effect in Tensile Loading

In all the experiments performed, ZnO nanowires demonstrated a steady increase in current (i.e. a decrease in resistance) with a specific voltage applied during loading, as depicted in Figure 3 (bottom).

In the case of non-piezoelectric materials, an increased electrical resistance can be expected in tensile loading as the diameter of the nanowire decreases and the length increases. In addition, the presence of a non-linear stress-strain curve in the elastic tensile regime of ZnO suggests that the mechanical response is affected by the piezoelectric effect of the nanowire. In a separate set of experiments, ZnO’s elastic modulus was calculated to be ~88 GPa. However, voltage was not applied in these experiments.

As shown in Figure 4 (top), resistivity was measured from the linear I-V curves as a function of tensile strain. In Figure 4 (bottom), the piezoelectric response curve reveals a fluctuation in stress produced by applied voltage steps, keeping the nanowire at a constant 2.2% true strain. Interestingly, it was observed that the change in stress produced by voltage fluctuation, Δσ(V), is dependent on the amount of tensile strain (_εT) applied to the nanowire, and this Δσ(V) is directly proportional to - _εT.

Aside from the strain-induced mobility improvement as seen in semiconductor materials such as silicon, another possibility is that tensile elongation influences the piezoelectric potential across the nanowire length, considerably modifying the electrical transport properties.

Figure 4. Resistivity calculated by linear I-V, inset shows a typical I-V sweep (top). Periodic stress generated by applied square-wave voltage signal from 0 V to 2.45 V while holding the nanowire at constant 2.2% strain (_εT) (bottom).

Conclusions

The in-situ electromechanical study has demonstrated an innovative method to comprehend the direct piezoelectric response of ZnO nanowire. Additionally, the - E-PTP technology can help understand the way phase transformation, defects, large-strain plasticity, size and crystallographic and grain boundary orientations influence both electrical and mechanical properties of nanomaterials.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

For more information on this source, please visit Bruker Nano Surfaces.