IEEE Consumer Electronics Magazine - March/April 2021 - 101

pressure applied. Sensitivity is one of the figures
of merit of the pressure sensor, which is denoted
by " S " and determined using the equation S ¼
(DI/I0)/(DP), where DI (final current - initial current) represents the shift in the sensor current
after applying a specific pressure and I0 is the
sensor current without any pressure applied.
Figure 1(b) displays the graph plotted between
DI/I0 and DP and the slope of this graph was considered as sensitivity which was found to be
approximately $ 0.29 kPa-1. The sensitivity of
the fabricated device is less compared to some
of the literature. But the fabrication techniques
utilized for such sensors are sophisticated and
expensive. Hence it is a cost v/s performance
tradeoff. Further, the fabricated device was subjected to a constant pressure of multiple cycles
approximately for 60 s and the results are shown
in Figure 1(c). From the graph, it was observed
that there is a sudden rise in the sensor current
immediately after applying external pressure on
the sensor. After releasing the pressure, sensor
current returns to the initial value. Response
time is one of the significant parameters of the
pressure sensor, which is defined as the time
taken by the sensor current to rise from 10% to
90% of the peak value. The rise time of the fabricated sensor was measured to be approximately
$ 200 ms, as shown in Figure 1(d).
Furthermore, the fabricated sensor was tested
under dynamic conditions by applying various
pressures and the corresponding temporal
response was given in Figure 1(e). As seen in temporal response, the fabricated sensor revealed variations in current levels proportional to the
applied pressure. As the applied pressure
increases, an increase in the current level was
observed, indicating that variations in the pressure values can be clearly differentiated by the fabricated device. To check the reliability, the sensor
was testing for 1000 continues cycles. As seen in
Figure 1(f), negligible change in the sensor current
was observed, suggesting that the high reliability
of the sensor and it can be used for multicycle
pressure sensing applications. For continuous
pressure monitoring, hysteresis is an important
parameter. Figure S3a, SI shows the hysteresis
graph of the sensor plotted between DR/R0 and
pressure applied. Low hysteresis of the fabricated
device reveals that the sensing mechanism can be

March/April 2021

ascribed to tunneling phenomenon instead of rupture/deformation in NiS flowers. To ensure that
the external bending does not affect the performance of the sensor, it was put through 500 bending cycles (both tensile and compressive) and the
corresponding graph between normalized sensor
resistance and bending cycles was drawn as
shown in Figure S3b, SI. Negligible deviation of
Æ2.6% in the sensor response, which reveals the
highly robust nature of the sensor.
The change in the fabricated device current
due to the applied pressure can be understood by
the following reasons. As seen in the high-magnification image (Figure S2c), NiS have a 3-D nanoflower structure having nanorods which looks
similar to Christmas-tree nanostructures. These 3D Christmas-tree nano structures are densely
packed and consists of many individual NiS nanorods as seen in high-magnification FESEM image.
When external pressure is applied on the sensor,
the distance between the nanorods changes significantly, which leads to the change in conductivity
of the sensor and it implies the rise in sensor current. Second, individual NiS nanorods are separated by a distance of $1-3 nm, as revealed from
the high-magnification FESEM images, which is a
tunneling distance. Because of the external pressure applied, nanoscopic gaps between adjacent
nanorods decreases, consequently lower tunneling resistance. The reduction in tunnel resistance
increases the tunneling current of the sensor.
Thus total sensor current increases due to applied
pressure. The fabricated device showed negligible
hysteresis [see Figure S3a] and it confirms that the
cracks or ruptures in the NiS nanoflowers deposited on cellulose paper are very minimal, which
can be ascribed to the remarkable mechanical
properties of NiS and the interfacial binding
between NiS and cellulose paper substrate post
vacuum filtration process. Therefore, instead of
the rupture mechanism, the tunneling phenomenon plays a significant role, and thus the accuracy
of the pressure sensor output does not deteriorate
considerably over time.
Inspired by the high sensitivity and large area
of the fabricated NiS/paper-based sensor, it was
utilized for lab-based smart floor sensing application. A large area (10 cm Â 4 cm) sensor was
fabricated by embedding the NiS/paper between
two thin PDMS films and digital image of the

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IEEE Consumer Electronics Magazine - March/April 2021

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