on 07-Mar-2018 (Wed)

When the scan is performed along the tube axis, we were able to de- tect IP signal due to a shear component of the piezo- tensor (d 15 ), corresponding to the polarization paral- lel to the tube axis, and OOP that reflects polarization along the tube radius (Figure 1b). The fact that we observed only the shear component (Figure 1c,d and 3) unequivocally suggests that the only polarization component existing in PNTs is along the tube axis

The bell-shape profile (Figure 2a,c) is at- tested to tubes with high outer to inner (r) diameter ratios (R/r) where the internal cavity does not signifi- cantly affect the E-field distribution and resulting pi- ezosignal (i.e., penetration depth of the E-field is com- parable or smaller than the wall thickness). To avoid the influence of any geometrical constraints on the cantile- ver torsion and IP signal detection, we always used the maximum contrast value for the analysis (arrow on Fig- ure 1d). In some cases, we observed a two-hump pro- file (Figure 2b,d) where the piezosignal dropped to about half of its maximum value. This situation reflects the distortion of the applied E-field due to the presence of the cavity

In order to understand the origin of piezoresponse, we should consider the possible d ij matrix for the space group P6 1 . 17 In this group, only two shear components exist, which apparently describe deformations along two perpendicular axes relative to the tube axis. In the configuration shown in Figure 1b, the component of the shear deformation is parallel to the tube axis and proportional to 1/2d 15 V ac cos ␣, where ␣ is the angle be- tween the tube axis and scanning direction and d 15 is the shear piezocoefficient

trast is completely reversed when the sample was physically rotated at 180° (cf. Figure 3a,b). Both fea- tures have contrast reversal and a predicted d 15 (␣) de- pendence point to the piezoelectric origin of the signal. This also rules out possible contribution of spurious electrostatic signal as it suggests only OOP compo- nent. 19 In addition, piezoresponse signal was indepen - dent of the vertical force (Supporting Information, Fig- ure S1), and force⫺distance curves for PNTs were identical to those taken on a rigid substrate (Support- ing Information, Figure S2) without any sign of mechan- ical indentation of the former. This excludes possible artifacts related to electric-field-controlled stiffness and slip motion of the tip

The piezoelectric anisotropy (i.e., the ratio between d 15 and d 33 ) is unknown for PNTs, but for a vast majority of ferroelectric materials, it varies between 1.3 (PZT) 29 and 10⫺12 (LNO). 26 It means that the expected value for the longitudinal piezocoefficient for PNTs (not accessible by our measurements) is between ⬇5⫺6 (as for LNO) and ⬇50 pm/V (as for PZT). In order to further prove the piezo- electric nature of the signal, we measured the ac voltage dependences of deformation for two oppositely oriented tubes (Figure 4a). As expected, two straight lines were ob- served without any nonlinearity or irreversibility even at a high ac voltage of 16 V (see also Figure S3 of Support- ing Information). It signifies that the observed piezore- sponse is very stable and PNTs can be driven under high excitation level. There was no visible degradation (within the measurement accuracy) of the topography after driv- ing during prolonged time (tens of minutes

Another striking feature is the observed dependence of the piezo- response on the tube diameter (Figure 4b). It is natural to assume that, for sufficiently thick walls, the piezo- response (shown on Figure 3a) will depend only on the piezoproperties of the PNT material underneath, rather than on the geometrical constraints

Comparison with well- known piezoelectric LiNbO 3 and lateral signal calibration yields sufficiently high effective piezoelectric coefficient values of at least 60 pm/V (shear response for tubes of ⬇200 nm in diameter).

The coupling between the piezoelectric and semiconducting properties of nanowires was discovered in 2006 [13], in which the strain-induced piezoelectric polarization was used to tune the conductivity of the nanowire. This effect was called the piezotronic effect, and has been investigated in piezoelectric, semiconducting nanomaterials like ZnO, zinc sulfide (ZnS), cadmium sulfide (CdS), indium nitride (InN), gallium nitride (GaN), and monolayer molybdenum disulfide (MoS 2 ) [14–20]. Mechanical stimuli are ubiquitous and abundant in the environment. The piezotronic effect combines piezoelectric polarization with semiconductor properties and allows the direct and active interaction between devices and stimuli. This new fundamental phenomenon continues to inspire novel device applications and has led to an emerging field called “piezotronics”

the adsorption of chemical species onto a semiconductor material acts as a gate voltage, tuning the channel conductivity across the bulk material. This phenomena has been widely used for chemical sensing [22,23]. When the sensor includes piezoelectric semiconductor materials and a Schottky junction at the metal–semiconductor interface, the voltage from the adsorbed species can alter the energy barrier height and regulate the carrier transport. At the same time, the barrier can also be tuned with the strain-induced piezoelectric potential

Nanostructure-based piezotronic sensors have attracted much attention because of their low power consumption and high sensitivity enabled by large surface area to volume ratio and novel piezotronic effect

Piezoelectricity exists in non-centrosymmetric crystalline materials. In a piezoelectric lattice, mechanical stress alters the distance between the center of positive charges and the center of negative charges, which creates electric dipole moments or changes the existing ones. In either way, polarization charges are induced on the surface of the material. Alternatively, an electric field can cause mechanical strain in the crystal

poly(vinylidene fluoride) (PVDF) [34], diphenylalanine peptide [35]

The piezotronic effect was first discovered in 2006 in piezoelectric ZnO nanowires with n-type conductivity in Wang’s group [13]. Generally, it exists in heterojunction systems with one material being a piezoelectric semiconductor and the other being a metal, a semiconductor, or an electrolyte [42]. When the piezoelectric material is deformed, polarization charges are induced at the junction between two materials, which modify the interfacial band structure and thus the carrier transport, trapping, generation, and recombination processes. In short, piezotronic effect is a change of the interfacial carrier dynamics due to the piezoelectric polarization.

As shown in Figure 1a, a ZnO wire has its crystallographic c-direction pointing left and two gold electrodes on its two ends. As-synthesized ZnO wires usually have n-type conductivity with intrinsic defects and impurities as shallow donors [43], and it forms Schottky barriers with high work function metals, such as gold. Schottky barrier is a rectifying contact. It allows only electron transport from ZnO to gold and prevents the electron transport from gold to ZnO. Two back-to-back Schottky diodes result in very little current flow. In Figure 1b, the ZnO wire is under tensile stress, and positive polarization charges appear on the left end and negative charges on the right. Polarization charges will be partially compensated by internal and external free carriers, but may not completely diminish due to the moderate doping level and finite charge screening lengths of electrodes [44]. At steady state, remnant piezoelectric charges still exist at the two contacts, and the electrostatic field from those positive charges reduces the Schottky barrier height, while negative charges raise the Schottky barrier height. The asymmetric barrier change makes it easier for electrons to transport from left to right. Similarly, a compressive stress induces piezoelectric charges with opposite polarities, and allows electron flow from right to left.

Several features of the piezotronic effect are emphasized here. First, fundamental theory shows that the change of the Schottky barrier height is proportional to the piezoelectric charge density, which is proportional to the strain. Because the current depends exponentially on the Schottky barrier height, the relationship between strain and current flow is also exponential [20]. Second, the piezotronic effect is not a transient effect. As long as the strain holds, remnant piezoelectric charges can stay at the interface and the piezotronic effect will not disappear [44], although some slight decay over time was also observed [21]. Third, the piezotronic effect is an interface phenomenon, and should not be confused with the piezoresistive effect. The piezoresistive effect describes a change of the electrical conductivity of a semiconductor or metal when strain alters its bandgap, and thus it is a volume effect. Usually, the piezotronic effect has a more significant influence on the current flow than the piezoresistive effect when a Schottky barrier exists [45]

Single nanowire devices allow for very high sensitivity, while nanowire arrays provide robust and stable devices due to the redundancy of multiple wires [50,54]. In the case of Liao et al. [56], ZnO nanowires were combined with carbon fibers to form a hybrid strain

Typically, nanowires are oriented laterally on a flexible substrate, with the bending applied perpendicular to the direction of the wire [21,51,58]. Uniquely, Zhang et al. makes use of arrays of vertical ZnO nanowires, perpendicular to the substrate, improving device resilience and achieving a gauge factor higher than previously reported for a single nanowire [21,50]. While lateral nanowires technically experience a bending load, it is often reasonable to assume that the nanowires are experiencing pure compression or tension, due to the geometry and mechanical properties of the device and substrate [58]. This results in a potential distribution along the c-direction of the nanowire, as opposed to across the diameter of the nanowire as in the case of lateral deflection.

A substrate, often a flexible material such as polyethylene (PET) or polystyrene (PS), serves as the base of the device [21,46,50,55–58]. A metal-semiconductor-metal structure is typically employed, with the metals serving as electrodes and forming either ohmic or Schottky contacts with the semiconducting nanostructure [21,46,50,51,53,55–58]. The Schottky contacts may be either symmetric or asymmetric, partially depending on the work functions of the chosen metals, commonly silver paste, gold, Indium-Tin-Oxide (ITO), or platinum [21,46,50,51,53,55–58]. As a final step, the nanostructures/devices are usually encapsulated in polydimethylsiloxane (PDMS) or other polymers to protect against contamination and corrosion

he current-voltage (I-V) behavior is commonly used to characterize piezotronic devices. Typical outputs are on the order of a few to tens of microamps for biases between −3 V and +3 V [21,46,50,51,53,55–58]. The metal-semiconductor-metal structure of the devices usually has double Schottky contacts with the n-type semiconductor, however, one of the contacts may be ohmic [50]. Devices fabricated with the same electrode metal at the source and drain electrode will usually have a symmetric I-V curve while devices with different metals will have asymmetric barrier heights and display rectifying behavior [50,56]. Charge transport at the barrier is commonly explained using the thermionic emission diffusion model

The nanowires, obtained under thermal evaporation process [77], are typically hundreds of nanometers in diameter and hundreds of micrometers long. Figure 7c,d shows the current response over time as glucose is added under 0.33% and 0.79% compressive strains, respectively. The current increases quickly after the addition of glucose. As the compressive strain becomes higher, this increase is enlarged, and the current level is also shifted to higher values. These improvements in current response indicate that the resolution and the signal to noise ratio of the sensor are enhanced by simply applying higher compressive strain to the nanowire

The observed increase of current by adding glucose is the result of higher charge carrier density produced by GOx-catalyzed glucose reaction with water and oxygen on the ZnO surface. In this reaction, H 2 O 2 is produced and injects electrons onto the n-type ZnO nanowire, increasing its conductivity [72]. This response can be further enhanced by strain through the piezotronic effect at the reversed-bias Schottky contact, where the Schottky barrier is dominant in the charge transport process throughout the device. With the c-axis pointing toward the source and the nanowire being under compression, non-mobile ionic positive charges accumulate at the drain and tune the Schottky barrier height at the interface between the n-type ZnO nanowire and silver. Since the current response is exponentially dependent on the barrier height, it can be significantly tuned by the applied strain to the high sensitivity range for glucose sensing as demonstrated by the experimental results