Negative Piezoresistive Effect in a Stretchable Device Based on a Soft Tunneling Barrier

Piezoresistive soft composite materials are widely used in strain sensing and typically exhibit a decrease in conductivity upon elongation—the so-called positive gauge effect. We demonstrate a thin-film architecture that features the inverse behavior: a strain-induced transition from insulating to metallic conductivity, spanning nine orders of magnitude in conductivity. Our approach is based on a nanometer-scale sandwiched bilayer Au thin film with a polydimethylsiloxane elastomeric barrier layer. Upon application of strain, the thickness of the thin soft barrier decreases because of the strain governed by the Poisson effect, followed by electron-tunneling currents through the barrier, forming an interconnected bilayer metal electrode. An extremely high on–off electrical conductivity ratio (~10 9 ) is observed over a wide range of working strains (as high as 130%), which mimics the ideal features of a mechanical-force-controlled electric transistor. This conceptual design strategy is expected to benefit a wide range of applications in which operation under minimal standby power could be an essential feature, such as in implantable soft strain sensors and in prosthetic long-term monitoring systems for detecting sudden a swelling/volume expansion of human body organs or blood vessels, thereby helping to avoid acute and severe syndromes. sensor performance in the ε = 0.1 strain regime. e, Resistivity response to stepwise increments of strain with different applied strain and time intervals, showing accurate and reliable strain-sensor performance. f, g Possible single strain gauge circuit with an nGF electrode, realizing near-zero-power standby monitoring. f , A digital image of the developed strain sensor integrated onto infrastructure. g , Measured drift current and accumulated power of an nGF strain sensor versus a normal strain gauge, plotted as a function of the standby monitoring time.


Introduction
State-of-the-art strategies to achieve flexible and stretchable sensor devices, which are strongly demanded for diverse applications such as flexible and highly deformable electronics and devices, 1,2 artificial skin, 3,4,5 implantable health monitoring sensors, 6,7 and soft actuators/robotics, 8,9 mostly rely on forming and breaking a percolated network of a conductive filler and thus always resulting in an increase in resistance under stretching. 10 However, for power-conserving applications in a long-term standby strain sensor, 11  Negative piezoresistivity has rarely been observed in condensed matter such as metals (e.g., Ni) and semiconductors (e.g., n-type Si, Si nanowires, and some two-dimensional semiconductors). 12,13,14,15 Practical applications of negative piezoresistivity in soft/stretchable devices is further limited by their high Young's modulus and extremely low maximum yield strength. 13,14,15 The fabrication of composite stretchable negative-piezoresistive materials by, for example, utilizing a negative-piezoresistive filler embedded in an elastic matrix has been suggested. 16,17,18 However, the increased interspacing of the filler upon application of strain within a certain range counteracts the resistance reduction of the composite and the nGF strain range is limited to extremely small strain variations (ɛ = ~%). 16,17 The low off-mode resistance is not favorable for low power consumption in the standby mode. In addition, the resistance modulation in the on-state (with strain) is limited by the elastic matrix of the surrounding insulator; therefore the performance of stretchable composite materials with negative piezoresistivity is hampered by a poor on-to-off ratio (10 2 -10 3 ) and a low operating nGF strain range.
One of the fascinating properties of soft and elastomeric materials is that, if elastomers are used as a tunneling barrier material between two electric terminals, the potential barrier can be tuned via quantum-scale thinning by macroscopic stimulation, i.e., by mechanical strain, analogous to the gate effect in transistors. The ability to achieve current modulation across many orders of magnitude and the fact that one key parameter is a mechanical property (i.e., a thickness related to barrier height controlled by strain) together promise untapped possibilities for a new strategy toward stretchable piezoresistive materials with negative piezoresistivity.
Here, we report a new design of a stretchable negative piezoresistive device using a bilayer metal thin film sandwiching an ultrathin soft insulator barrier layer. Upon application of strain, the conductivity between the electrically contacted bilayer metal thin film undergoes a superior transition from =0 ≈ 10 −5 Ω −1 cm −1 at zero strain to =1.0 ≈ 10 5 Ω −1 cm −1 at unity strain. The very high off-state resistivity, the wide working range of negative piezoresistive strain, and the high conductivity in the on-state exceed the properties of all known stretchable conductors [19][20][21][22][23][24][25] and negative piezoresistive materials. 10 To illustrate the versatility and applicability of this new negative piezoresistivity concept, we demonstrate a mechano-gated stretchable electrical switch device and a zero-standby-power strain sensor.
This work paves the way toward numerous stretchable-strain-sensor applications that rely on long-term standby-mode strain monitoring with minimal power consumption.

Design of the sandwiched bilayer metal thin film with a soft thin barrier layer
To achieve stretchable negative piezoresistivity, a material must exhibit not only superior stretchable conductivity in the highly stretched state but also insulating behavior in its initial state; thus, a key challenge is developing new materials whose conductivity rapidly increases upon application of mechanical strain. We started our work by depositing a metal thin film onto an elastomer substrate via the physical vapor deposition (PVD) method. The relatively brittle metal thin film readily formed many cracks and lost its electrical interconnectivity once the underlying elastomer substrate was stretched; the metal thin film deposited onto the elastomer substrate showed a positive gauge factor (pGF) ( ≫ 0 ) (Supplementary Fig. 1 and scheme in Fig. 1c). Our new proposed design is composed of bilayer metal thin films with a thin soft intermediate layer placed between the metal layers as an insulating tunnel barrier. The layers are sequentially stacked on the substrate (see details in Methods and Supplementary Information S1 and Supplementary Fig. 2). The structural properties of the bimetal and soft-barrier thin-film device are shown in a cross-section dark-field scanning transmission electron microscopy (STEM) image (Fig. 1a) and in elemental line profiles of Si and Au core emissions obtained by energy-dispersive X-ray spectroscopy (EDS) (Fig. 1b). As discussed in the next section, the thin soft elastomeric barrier layer decreases in thickness because of the Poisson effect, which is expected to affect the interconnectivity between the metal bilayers upon the application of external strain. The PVD method enabled precise control of the thickness of the metal bilayers, which allowed us to exploit the percolation threshold at which the transition from the conductive to the nonconductive state of a thin metal layer occurs. Hence, by optimizing the deposition thickness (percolation engineering) and by controlling the deposition conditions of the soft tunneling barrier layer, we systematically obtained a stretchable electrode possessing all three modes of a stretchable gauge factor: positive, zero, and negative ( Fig. 1c-e). In particular, such an optimized design enables, for the first time, the fabrication of a high-performance negative-piezoresistive stretchable device and can provide an excellent platform for developing multimode piezoresistive strain sensors suitable for applications that require responsiveness to a wide range of strain in a single system.

Mechanism of nGF: Percolation engineering and quantum thinning
To study the effect of metal-layer thickness on the nGF property, the resistivity change at applied strains (0 ≤ ≤ 0.31) was plotted for various deposition thicknesses of both layers (the first metal layer, 1st , and the second metal layer, 2nd ) as a contour plot in Fig. 2a. We categorized the deposition thicknesses into three classes according to the different tendencies observed: "A" ( 1st < 25 nm, 2nd < 17.5 nm), "B" ( 1st > 32.5 nm, 2nd > 25 nm), and "C" (25 nm < 1st < 32.5 nm, 17.5 nm < 2nd < 25 nm). The corresponding resistivity changes for the full range of strain (0 < < 1.0) and the change rate,  Fig. 4), which implies that the structure is not only useful for attaining a high nGF but is also suitable for use in a wide range of soft electronic applications. For example, with a simple variation in the thickness of the metal layers, the resultant zero Gauge Factor (zGF) property could make the structure applicable as a stretchable interconnector with stable and superior stretchable conductivity (also utilized in the following demonstration subsection). As a result of the optimization of the deposition thickness (percolation engineering as scheme in Supplementary Figs. 5 and 6), the structure is electrically isolated at the beginning as a consequence of the intraconduction path being disallowed because of the insufficient deposition thickness and because the soft barrier layer with a proper high potential barrier is sufficient to prevent a tunneling current between the metal layers ( Fig. 3a). Upon the application of strain, the thickness of the barrier layer begins to decrease, whereas its length increases along the strain direction. Although the gaps between the intrametal domains in both layers may also increase, the thinning is expected to lead to a tunneling current across them.
Note the different current scales for different applied strains. Therefore, the generalized equation for the tunneling current can be simplified to 27 where s( ) is the barrier thickness dependent on strain and m is the electron mass. We assume that, for moderate strains ( < 0.5), the thin PDMS layer is bonded to the substrate and that its change in relative thickness follows the substrate deformation characterized by the TDS-THz measurements. The initial thickness (without strain) was set to 12 nm; this value was measured by ellipsometry on a model sample on a Si-wafer substrate (see details in Supplementary Information S3 and Supplementary Fig. 9). The calculated conductance of the nGF device is shown in Fig. 3e together with the measured values. The barrier height was set to = 2.3 eV to achieve the best agreement with the experimental data. Obviously, for > 0.5, the conductance saturates and does not demonstrate the model's prediction of exponential growth with increasing strain. We speculate that, at such large strain, the thin PDMS barrier layer begins to slip and loses its bonding with the substrate. As a result, the barrier thickness does not vary as the applied strain is increased further, leading to the observed current saturation. Presumably, the lateral conductivity (via intraconduction) also must change at such large strain regions. For other future material configurations, as evident from Fig. 3g, both the initial thickness of the barrier layer (s0) and the barrier height (φ) between the metal/barrier interface jointly determine the negative gauge factor: larger nGF values are expected for thicker and higher barriers. However, an excessively thick barrier in either height or initial thickness could give rise to a huge resistivity, which implies a certain limitation to the expected nGF value for this design.

High performance of negative piezoresistive and demonstration of a zero-power strain sensor
We compared the electromechanical performance of our device with those of state-of-the art, stretchable piezoresistive composites. Although stretchable piezoresistive composite materials have been widely developed for various working strains with a wide range of GF values, the GF values are all positive (Fig. 4a). Hence, inevitably, all stretchable devices have a finite resistance in the absence of strain and are not suitable for low-power standby strain sensor applications. By contrast, the GF of our nGF piezoresistive elastomer shows an average nGF of −14.5, which is comparable to the previously reported values; however, these previously reported values have only been reported in a small number of papers. 16,17 Notably, the negative piezoresistive range of our sample extends to 130% strain, and no pGF features are observed at any applied strain.
We also compared the strain-dependent absolute DC conductivity of our nGF piezoresistive device (Fig. 4b,c), which can be the most important characteristic for realizing a zerostandby-power strain sensor via low current flow in the standby mode. When the electrode is in the "off" state (i.e., in the absence of strain), the initial conductivity of our device is almost outside the measurable range ( =0 = 10 −5 Ω −1 cm −1 ); when the electrode is in the "on" state (in this case, ε > 50%), the conductivity even exceeds the performance of other stretchable electrodes reported elsewhere. [19][20][21][22][23][24][25] Electrical conduction in our structure in the stretched state can occur between two pure metal thin films without any interruption from the insulating matrix, resulting in superior on-state conductivity ( =1.0 = 10 5 Ω −1 cm −1 ) comparable to the conductivity of pristine bulk metals (e.g., Ag and Au, > 10 6 Ω −1 cm −1 ).
Note that this value is superior to any other reported value of stretchable electrodes in both of electrical conductivity and stretchability. As a straightforward demonstration, we visualized the pGF, zGF, and nGF performances with three of our selected devices (Fig. 5). Their high- on-to-off ratio was constant from the first to the final cycle, which revealed reproducible and reliable strain detection ( Fig. 6d and Supplementary Fig. 10).
To demonstrate the potential applications of the nGF behavior, on the basis of the results in Fig. 2, we fabricated a stretchable strain sensor by incorporating both the zGF (for a stretchable interconnector) and nGF (for the area of the sensing grid) features on a single device (a digital image of the as-fabricated device in Fig. 6a,b is shown in Supplementary Fig.   11). The extremely high resistivity in the sensing-grid region contributed to an almost zero current draft across the whole circuit, which means that power consumption in standby mode in the absence of strain was reduced ( Supplementary Fig. 12). This power-saving ability of the nGF piezoresistive elastomer makes the sensor highly suitable as a long-term strain monitor on massive infrastructure ( Fig. 6f and Supplementary Fig. 13). In comparison to a normal strain sensor (HBM, 1-LY11-6) that shows a constant current flow of ~1.2 mA in standby mode (see brown line in top of Fig. 6g), the nGF strain sensor showed no current flow (~0 mA; black line in top of Fig. 6g). Thus, the accumulated power consumption was maintained at almost zero in the standby mode (black line in bottom of Fig. 6g). If the duration of the standby mode becomes longstanding (~ years), these differences will be even more prominent. Thus, this lightweight and morphable nGF strain sensor could be very effectively implemented in long-term standby mode to detect early signs of collapse and sudden change in the systems spanning from organs in body to massive structures due to earthquakes.
In  zGF; e, nGF). In particular, for the nGF property, an extremely high on/off electrical conductivity ratio was obtained over a wide range of working strains.     The active sensing grid comprises an nGF electrode (with thickness combination "C" in Fig.   2a), whereas the pad and connector are realized by a zGF electrode (combination "B" in Fig.   2a). Scale bar denotes 1 cm and 1 mm in Fig. 6a and Fig. 6b Au thin-film deposition The PDMS substrate was mounted onto a sample holder for PVD (chamber: Hex, Korvus Technology) and a Au thin film was deposited by thermal evaporation. The deposition rate was 0.3 Å s −1 with the deposition chamber evacuated to a pressure less than 6.2 × 10 −6 mbar. The thickness was monitored in situ by quartz crystal microbalance.

Soft tunneling barrier formation
After a Au thin film was deposited onto the PDMS substrate, the sample was stretched to 80% strain to induce cracks in the Au thin film. The sample was kept under a reduced pressure of