Negative differential resistance in nanoclay films offers pressure sensingcharacteristicsNisha Pawar, H. B. Bohidar, Aakanksha Sharma, and Subhasis Ghosh Citation: Appl. Phys. Lett. 102, 103109 (2013); doi: 10.1063/1.4795524 View online: http://dx.doi.org/10.1063/1.4795524 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v102/i10 Published by the American Institute of Physics. Related ArticlesConductivity and frequency dependent specific absorption rate J. Appl. Phys. 113, 074902 (2013) Effects of hydrodynamic interaction on the equivalent conductivity minimum of electrolyte solutions in solvents oflow dielectric constant J. Chem. Phys. 138, 024503 (2013) Effect of pressure on the ionic conductivity of Li+ and Cl− ions in water J. Chem. Phys. 137, 144506 (2012) Eigen-like hydrated protons traveling with a local distortion through the water nanotube in new molecular porouscrystals {[MIII(H2bim)3](TMA)·20H2O}n (M = Co, Rh, Ru) J. Chem. Phys. 137, 144503 (2012) Transport properties of room-temperature ionic liquids from classical molecular dynamics J. Chem. Phys. 137, 044508 (2012) Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors

Negative differential resistance in nanoclay films offers pressure sensingcharacteristics

Nisha Pawar,1,2,a) H. B. Bohidar,2 Aakanksha Sharma,3 and Subhasis Ghosh3

1Experimental Physics-I, University of Bayreuth, Bayreuth-95440, Germany2Nanomaterials and Nanocomposites Laboratory School of Physical Sciences, Jawaharlal Nehru University,New Delhi-110067, India3Electronic Materials and Device Laboratory School of Physical Sciences, Jawaharlal Nehru University,New Delhi-110067, India

(Received 23 January 2013; accepted 4 March 2013; published online 14 March 2013)

Thin fluid films of discotic nanoclay particles dispersed in water were observed to exhibit anomalous

current-voltage (I-V) characteristics at room temperature. The I-V characteristics exhibit a transition

from space charge limited conduction to negative differential resistance on crossing a certain bias of

20 V. Here, the maximum current (Imax) found to be around 20 lA and Imax was found to scale with

applied pressure (P) following empirical relation Imax� P1/4 indicating that these films can be used as

pressure sensors. VC 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4795524]

Laponite1–5 is an interesting inorganic clay having a

disc shaped morphology with 30 nm diameter and 1 nm

thickness. It has anisotropic distribution of charge, which

propel it to show some unusual properties. The anisotropic

face rim interactions and discotic shape of nanoclay pro-

duces a very rich phase diagram.1 Generally, clays exhibit

ionic conductivity. They do not exhibit electronic conductiv-

ity. Because they reveal a wide electronic band gap of sev-

eral electron volts.6 But the intrinsic anisotropy arising from

geometrical shape and large dipole moment associated with

these nanoparticles motivated us to probe the transport

behavior of these particles in the presence of electric field

and pressure. Recently, a report7 in Ground Engineering dis-

cussed the importance of electrical properties of clay in

terms of dependence of their resistivity on moisture content

in the soil.

Herein, we report the observation of negative differen-

tial resistance (NDR) behavior at 20 �C in Laponite nanoclay

particles in an aqueous film and extend it to show that this

property can be exploited to generate a pressure sensor de-

vice. Nonlinear transport property of nanoparticles is suitable

for device applications in fast switching, logic circuits,

etc.8–10 However, no NDR was ever reported in nanoclay

films, which make the current work unique and interesting.

Laponite RD was purchased from Southern Clay

Products (USA) and used as received. The chemical structure

of Laponite is shown in Fig. 1. The powder material was dis-

persed (concentration¼ 1.5% w/v) in deionized water at

room temperature (20 �C) and mixed for 2 h using a mag-

netic stirrer to generate an optically transparent and homoge-

neous dispersion. A thin liquid film was created by drop

casting 20 lL of prepared dispersion on the substrate. The

films were annealed at different temperatures in the range

30–70 �C for 1 h. Pressure on the films was maintained by

allowing flow of nitrogen gas at controlled rate into the sam-

ple chamber. The size and shape of nanoclay particles on the

thin film were analysed by SEM Zeiss EV040. X-ray diffrac-

tion (XRD) studies were carried out on a PANalytic X’pert

PRO diffractometer. Raman spectra were recorded using FT-

IR/Raman spectrometer attached with a microscope (Varian

7000 FT-Raman and Varian 600 UMA).

Surface morphology of thin films was characterized by

Park System XE-70 atomic force microscope (AFM).

Microscopic changes in the thin film with electric field were

monitored using XE-70 Optical Microscope. The current-

voltage (I-V) characteristics of two terminal devices, which

were fabricated in planar geometry configuration and con-

sisted of two Au electrodes deposited on a glass plate, sche-

matically shown in Figure 1 inset. Two Au electrodes in

planar configurations were fabricated using shadow masking

technique. The channel length and width were fixed at 50 lm

and 3 mm, respectively. Thin film of Au (100 nm) was

evaporated on cleaned substrates using vacuum evaporation

technique under a base pressure of 2� 10�6 Torr and at the

rate of 1.0 A/s. I-V measurements were carried out using

Keithley pico-ammeter and Agilent voltage source.

Figure 2 shows the SEM of the nanoclay particles in the

thin film. Some of the particles form cluster due by face-

edge interactions among the particles.

Figure 3 depicts the I-V characteristics of films that were

exposed to different annealing temperatures. The I-V charac-

teristics can be divided into two regions: (i) when V< 20 V,

current increases with bias quadratically following I�V2,

showing SCLC11,12 (space charge limited conduction) region

(ii) when V> 20 V, current decreases with bias showing a

NDR region. The current follows non-monotonic depend-

ence on bias reaching a maximum current (Imax) of 20 lA at

V¼ 20 V. Figure 2 also shows similar behavior of I-V char-

acteristics for samples annealed at different temperatures.

The Imax for samples annealed at temperatures 30�, 50�, and

70 �C are 25, 20, and 18 lA, respectively.

NDR effect has been observed in many molecular and

organic electronic devices containing nanoparticles in the

past.13–17 In our system, the charge carriers transit the dis-

cotic particles that are plausibly arranged as filaments. It is

widely believed that selective trapping of charge carriers at

a)Author to whom correspondence should be addressed. Electronic mail:

nisha.pawar@uni-bayreuth.de. Tel: þ49 (0)921 55-2506. Fax: þ49 (0)921

55-2521.

0003-6951/2013/102(10)/103109/5/$30.00 VC 2013 American Institute of Physics102, 103109-1

APPLIED PHYSICS LETTERS 102, 103109 (2013)

the metal nanoparticle/organic film interface17–20 gives rise

to NDR behavior. We argue that the clay particles arrange

through rim-edge binding to form conducting filaments

shown schematically in Figure 4.

Since the edges are positively charged, these act as elec-

tron traps during the conduction process. Thus, it appears

that hopping of charge carriers on the connected filaments

may be responsible manifesting the NDR behaviour.

At higher bias, conduction is retained at the cost of loss

of many filaments. Under this condition, the filaments break

due to the viscous drag experienced as the film flows towards

the anode plate. More and more filaments get disrupted as

the applied field increases leading to the loss of conductivity,

as was noticed in our experiments. The hydrostatic flow of

the film was visualized through the optical microscope as

function of applied bias. The movement (or realignment) of

the liquid meniscus in the film is clearly seen in the micro-

scope images of Figure 4 with applied bias. The solvent

(shown in red) provide a medium for the movement of par-

ticles with bias. As shown in this figure, there is a net flow

from negative to positive terminal with applied bias. When

there was no bias applied, the clay particles were randomly

dispersed, however with increase in bias, the current

increased as electric field drove the alignment of particles

(second column of Figure 4). Hence, it can be concluded that

nanoclay platelets aligned in the direction of applied field (as

filaments) and the current increased with increase in the field

until the threshold bias voltage �20 V was reached. For bias

voltage exceeding 20 V, solvent along with the dispersed

particles (present as filaments) crossed the channel (third col-

umn of Figure 4). During this process, many filaments get

ruptured, thereby causing a drop in current. This behavior

was quite reversible for the fluid films annealed to lower

temperatures. Because the presence of large amount of sol-

vent in such films provided a medium to heal the damage

caused by high bias. The presence of solvent facilitated the

reversible movement of particles on reversing the bias

voltage.

It can be inferred from the results that the dispersing sol-

vent and the anisotropic distribution of charge on the clay

particles are playing important role in determining the non-

linear I-V characteristics seen as NDR effect in these films.

In order to identify the role of solvent, it was felt imperative

to examine the water content of these films using Raman

spectroscopy. Figure 5 shows the variation in fractional peak

area for different Raman modes (in the Raman spectra

recorded for the thin film of nanoclay) as function of anneal-

ing temperature. The peak around 3200 cm�1 in the Raman

spectra arises from the in-phase vibration of OH stretching

mode, structurally arranged water which is sometime

referred as ice-like structure. Peak at 3310 cm�1 signifies the

OH stretching in partially structured water and the peak cen-

tered around 3460 cm�1 arises from weakly H-bonded water

molecule, amorphous water. The fractional area under the

peaks 3200 cm�1 was observed to remain invariant with

FIG. 1. (A) Schematic representation of device

structure (B) molecular structure of Laponite

(nanoclay).

FIG. 2. SEM image of the nanoclay particles in the thin film.

FIG. 3. I-V characteristics of films exposed to different annealing tempera-

tures under a pressure of 50 mbar. Note the existence of two distinct conduc-

tion regimes that differentiate the explicit dependence of current on bias

voltage. Inset depicts the I-V response in the low bias region where SCLC

was noticed. The solid line in the inset is fitting to data, which yields I�V2.

103109-2 Pawar et al. Appl. Phys. Lett. 102, 103109 (2013)

change in temperature while the same for 3310 cm�1 and

3460 cm�1 band decreased with increase in annealing tem-

perature (Figure 5). This indicates significant loss of solvent

(water) from these films. Realize that particle hydration is

provided by mostly partially structured water. Thus, loss of

hydartion would manifest in the breaking of filaments pres-

ent across the channel resulting in drop in current.

This also explains the reduction in the observed maxi-

mum current Imax with annealing temperature due to the

lesser amount of solvent required for healing against ruptur-

ing the filaments caused by applied bias. This infers that the

annealing temperature controls the amount of solvent in the

film, which plays an important role in the movement of par-

ticles across the channel attributing NDR feature to the film.

These films were subjected to XRD analysis in order to

probe their micro-crystalline characteristics. A representative

set of data is shown in Figure 6 for a film annealed at 30 �C.

XRD and AFM topographic images of the thin films have

been measured in-situ under electric field. Figure 6 clearly

shows how the peak intensity of XRD and film roughness

calculated from AFM topographic images vary under electric

field. For fields below 20 V, the XRD peak intensity

remained invariant under applied bias but a sharp decrease

was noticed when the voltage crosses 20 V (in NDR region).

Thus, the film characteristic is found to alter significantly as

FIG. 4. Comparison of optical (middle row) and morphological (bottom row) changes of a nanoclay film (annealed at 30 �C) in different bias regimes (i) no bias

(column 1), (ii) SCLC (column 2), and (iii) NDR (column 3). Here, "þ" and "�" sign show respective electrode positions. Top row shows the schematic represen-

tation of particle arrangement in three regimes. Middle row shows the microscope images of the film of nanoclay particles. Yellow line shows the channel (50 lm

and 3 mm) with þ and � as the electrode on the sides shown, red color shows the movement of the particles in the film as a function of bias applied. Last row

shows the AFM snapshots on the channel of the film. The first column shows the random alignment of the particle and their cluster in the absence of any bias volt-

age, second column shows the alignment of the particle forming a filament across the channel, and third column shows that filament got broken after the applica-

tion of higher bias voltage (or the liquid film shown by red color migrates to the anode plate). Upper row is the middle row rotated by 90 �C.

FIG. 5. Variation in peak area for three Raman bands at 3200 cm�1,

3300 cm�1 and 3460 cm�1 as a function of annealing temperature.

103109-3 Pawar et al. Appl. Phys. Lett. 102, 103109 (2013)

function of applied bias, a property that was exploited subse-

quently. The change in XRD peak intensity and surface

roughness clearly indicate the change in the alignment of

clay particles embedded in the film with bias. Interestingly,

the average roughness of the film was observed to decrease

with the applied field and passing through minima at �20 V.

The correlation between film roughness and XRD inten-

sity clearly indicates the reduction in alignment of clay par-

ticles resulting a drop in current and the appearance of NDR.

Presumably, in the NDR region, movement of solvent medi-

ated clay particles creates various trap states, which provide

hindrance to the flow of current. However, the morphologi-

cal changes in the samples were not visible at any instance.

The unique field dependent characteristic shown in

Figure 5 was effectively used to propose the design of a

potential pressure sensor. Figure 7 shows the I-V characteris-

tics of a film exposed to different pressure. The current pro-

files shown in Figures 3 and 7 are similar except Imax

increases with pressure. This implies, as discussed before,

that the particle arrangement inside the film is sensitive to

applied pressure. The maximum current is plotted as function

of applied pressure in the inset of Figure 7, which depicts

excellent pressure sensitivity. At low pressure, the particles

have random orientation, which results in the flow of less

current due to misalignment of particles. Arrangement of

particles, presumably as filaments was found to be facilitated

at higher pressure. The maximum current, Imax, determined

from Figure 7 was fitted to the applied pressure, P, which fol-

lowed a power-law scaling relation given by Imax� P1/4. The

observed behavior was completely reversible with a resetting

time �20 s. Thus, the observation of SCLC to NDR cross-

over in Laponite films revealed a unique and interesting elec-

trical property that was not known hitherto. This particular

attribute has potential to be developed in to a nano material

based device.

In conclusion, we show that nanoclay films exhibit

unique bias dependent conduction behaviour not known

hitherto. Conduction through films is governed by SCLC for

low bias, whereas above a threshold bias, the film shows

NDR revealing a non-monotonic dependence on electric field.

The dependence of Imax over annealing temperature infers the

alignment of particles mediated by solvent. It was conjectured

with the help of AFM and XRD studies that linear organiza-

tion of the anisotropic clay platelets in the form of filaments

provides the conduction path. The rupturing of filaments at

higher electric filed is responsible for NDR. The exact nature

of charge carriers remained elusive and undetermined in our

studies that are a subject of future work. These films depicted

considerable pressure sensitivity, a feature that will enable

these materials to be developed as pressure sensors.

We are thankful to Advanced Research Instrumentation

Facility of the University for Raman, SEM, and XRD facil-

ity. N.P. acknowledges support from Council of Scientific

and Industrial Research, Government of India for fellowship.

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