Quantum tunneling sensors can be used for force sensing and biomolecule detection. Force-based quantum tunneling sensors utilize the principle that tunneling current increases exponentially as the width of the tunneling barrier decreases with applied force. This phenomenon is exploited in quantum tunneling composites which contain conductive nanoparticles in a flexible polymer and can be used to create sensitive touch screens and force sensors. Biomolecule sensing uses quantum tunneling to detect different molecules by how much they alter the tunnel current when introduced into a steady-state tunnel junction. These techniques open applications in healthcare diagnostics, smart skins and other high-precision force sensing areas.

1. Quantum Tunneling Sensors
SK2901 – Quantum Materials and Devices
Jaskaran Singh Malhotra
Aniruddha Paul

2. Contents
• Wave nature of Matter
• Quantum Tunneling (QT)
• Applications of Tunneling
• Force based QT sensors and applications
• Biomolecule sensing using QT

3. Wave Nature of matter
• Matter has both particle-like and wave-like nature (De Broglie,
1924)
• The wavelength (λ) associated with a matter wave is related to it’s
momentum (p) or mass (m) & velocity (v) as -
λ =
ℎ
𝑝
=
ℎ
𝑚𝑣
where h = Planck’s constant
• This wave nature becomes significant for light weight particles
(like electrons)

4. Wave nature of matter
• Schrödinger’s equation* is used to describe the wave function of a
particle –
−
ħ2
2𝑚
𝑑
𝑑𝑥2 Ψ(𝑥) + 𝑈(𝑥)Ψ(𝑥) = 𝐸Ψ(𝑥)
• The wave function is a function of space here and the square of the
wave function Ψ(𝑥) 2
gives the probability of finding the particle at
the particular point in space
*here time independent equation is shown

5. Quantum Tunelling
• When there’s an energy barrier (V>E)
in a region of space, the particle
cannot cross the barrier (Classical
mechanics)
• But as a consequence of wave nature,
the solutions for the Schrödinger’s
equation for a particle in such a
situation gives a decaying wave
function
• Thus, there is a probability that a
particle will tunnel through the barrier.
source : https://www.testandmeasurementtips.com/quantum-
tunneling-and-tunnel-diodes/
a
V

6. Quantum Tunelling
• Tunneling (or transmission) probability across a square barrier
𝑇 = 1 +
𝑠𝑖𝑛ℎ2
(
𝑎
ħ
2𝑚(𝑉 − 𝐸))
4
𝐸
𝑉 1 −
𝐸
𝑉
−1
• In case of tunneling electrons (or charged particles), the current
is directly proportional to the amount of charge carriers and
hence the tunneling probability
• Marked in red, the width of the barrier (a) and the energy of the
barrier (V) are crucial aspects that can be tuned for a functional
device that uses tunneling effect

7. Use of tunneling phenomena
• STM – Scanning Tunneling Microscope
utilizes the quantum tunneling to study surface
morphology at atomic level. The tunnel current
between probe tip and the surface atoms
determines their position relative to the tip and
helps in imaging the surface.
• Tunneling diodes – Heavily doped p-n junctions
of a small width (~10nm) where electrons tunnel
through the junction to get aligned with the empty
hole states. Increasing voltage across it, reduces
the tunnel current initially (negative resistance
region) and after a certain threshold voltage, the
junction behaves like a classical p-n junction
*source :
https://www.nanoscience.com/techniques/scanni
ng-tunneling-microscopy/
*source :
http://www.wikiwand.com/en/
Tunnel_diode

8. Quantum Tunneling Sensors
2 kinds -
• Pressure/Force based sensors – based on the
increase in tunneling current (reduction in
resistance) upon application of pressure which
essentially changes the barrier width
• Biomolecule detection sensors – based on the
change in tunnel current as different molecules
(having different energy barrier) are introduced
in a fixed dimension of region in space
𝑇 = 1 +
𝑠𝑖𝑛ℎ2
(
𝑎
ħ
2𝑚(𝑉 − 𝐸))
4
𝐸
𝑉
1 −
𝐸
𝑉
−1

9. Touch/pressure sensors
• A Quantum tunneling sensor utilizes a
Quantum tunneling composite.
• Tunneling probability (or tunnel current)
increases exponentially as the width of the
barrier is reduced (or Force is increased).
• Thus, the resistance of the barrier decreases
exponentially with applied force.
*source :
Amarsinghe et al
(2013)
*source :
http://practicalphysics.org/qt
c-%E2%80%93-discovery-
novel-material.html

10. Quantum Tunneling Composites (QTCs)
• A Quantum tunneling composite is a material
comprising a non-conducing elastomeric binder
and a conductive metal
• The material was developed by David Lussey in
1996 and is currently patented by his company
PeratechTM
• Commercially available as QTC pills – variable
resistors that change resistance exponentially to
the force applied
• The conductive nanoparticles (nps)have spikes
over the surface and are incorporated into the
flexible elastomer.
• As the elastomer is compressed, these
conductive nps come closer and conduct
electricity through tunneling (not physical
touch)*
source :
http://informationdisplay.org/id-
archive/2012/january/technology-
preview-quantum-tunnelling-
composite-t
source : Azsman et al
(2016), ICSGRC
*confirmed by Prof. David Bloor http://informationdisplay.org/id-archive/2012/january/technology-preview-
quantum-tunnelling-composite-t
QTC pill

11. Advantages
Quantum Tunneling
Composites
● Surface can be flexible
● Adds an extra dimension
● Easy integration
● No spacer/air gap
● Very sensitive and can be used
with gloves.
Based on quantum tunneling
between nanoparticles in a gel.
Capacitive touch
● More power consumption
● Cannot be used with gloves
● Humidity is a problem
● implementing on large areas is
expensive
Based on change in capacitance
on screen when touched as
human body is conductive.
Resistive Touch
● No multi touch
● Soft screen, prone to damage
● less sensitive
Based on contact of two
conductive layers separated by
spacers.

12. Applications
● Touch screen- apple patent (2017), qtc whiteboards, sensors for automotive
screens
● Force sensor- Traffic monitoring, accelerometers, concrete composites*.
*B. Han, B. Han, X. Yu, “Effects of content level and particle size of nickel powder on the piezoresistivity of cement-based
composites/sensors,” Smart Mater. Struct., vol. 19, pp. 065012, Apr. 2010.
https://www.peratech.com

13. Applications
http://informationdisplay.org/id-archive/2012/january/technology-preview-
quantum-tunnelling-composite-t
● Smart skin- Tactile sensors,
haptics for robotics/prosthetics
● Smart clothes- flexible pressure
sensors, implemented in controls
for ipads, sports monitoring,
impact assessment in physical
sports, pressure distribution in
walking and physiotherapy.
*] R. S. Dahiya and M. Valle, “Tactile sensing for robotic applications,” Sensors Focus Tactile Force Stress Sensors, pp. 289–304,
2008
**A. D. Lantada, “Quantum tunnelling composites: Characterisation and modelling to promote their applications as sensors,” Sensors
and Actuators A: Physical, vol. 164, no. 1–2, pp. 46–57, Nov. 2010..

14. Challenges with QTCs
● Thermal Expansion causes metal particles to separate at the core.
● Rise in temperature leads to reduced viscosity.
● It takes considerable time to return to its original state.
● QTC-based sensors have been tested to operate from ‒40°C up to
100°C and weight range of 0.1N to 20N.

15. Quantum tunneling biomolecule sensing*
• Maintain a steady bias voltage and
distance at a tunnel junction
• The tunnel current is dependant on the
medium of the tunnel junction
• Introducing a biomolecule in the
junction essentially changes the junction
barrier and hence changes the tunneling
current.
* Albrecht (2012), Nature Communications

16. Quantum tunneling biomolecule sensing*
• This can be used to detect different
molecules based on how much they
alter the tunneling current
• The biomolecules chains (e.g. DNA /
RNA / proteins) are introduced in and
out of the junction one molecule at a
time
• Each biomolecule will alter the tunnel
current differently and this is used to
identify it.
* Albrecht (2012), Nature Communications

17. Thank you for your attention