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Negative Differential Conductance Materials
for Flexible Electronics
Samuel Littlejohn, Ashok Chauhan & Alain Nogaret
Department of Physics, University of Bath
Giles Prentice & Dan Pantos
Department of Chemistry, University of Bath
Atlas-Elektronik , October 2 Oct 2013
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
Flexible vs rigid electronics
Transistors
NDR devices
Opto-electronics
Rigid substrates
Field Effect Transistor
Bipolar transistors
Esaki diodes
Semiconductor
superlattices
Resonant tunnelling
diodes
Light emitting diodes
Optocouplers
LASER heterojunctions
Flexible substrates
Organic transistors
(pentacene, PEDOT,
graphene …)
In plane transport:
Polymer: p-type conduction, oxidation, low mobility,
finite lifetime …
Hybrid: Island-bridge approach (Rogers group)
Graphene: finite OFF current (no band gap).
Tunnelling transport
> This Talk <
Perpendicular transport:
Oscillators, Amplifiers, multistate
memories…
Organic
electroluminescent
devices
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
Negative differential resistance in organic materials
Metal Polymer NDR
Moller et al., Nature 426, 166
(2003);
Gregor Thin Solid Films (1972)
Mechanism: Oxido-reduction of
conjugated polymer
Graphene/ h-boron nitride
Britnell et al., Nature Comm.
(2013)
Mechanism: In-plane momentum
conservation & electrostatic
feedback
Graphene bilayer
Massum-Habib et al., APL 98,
192112 (2011)
Mechanism: electric field
induced energy band gap
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
Preparation of composites
Graphitic nanoparticles:
HOPG nanoparticles: 450nm
AC amorphous carbon: 50nm
PC Pyrolytic carbon: 50nm
Preparation:
• Silicone matrix (RTV 139).
•
•
•
•
•
•
•
Mixing graphitic NPT in silicone.
Add naphtalene diimide
molecules.
lower the graphite silicone
tunnel barrier.
increase conductivity.
Add Catalyst (C138).
Vulcanization at 300K (24hrs).
125m thick composite film
Sublimate Au contacts.
100 contact resistance.
Miscibility threshold: 32% vol.
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
Current-voltage
characteristics
of of
graphitic
allotropes
in silicone
Proof of principle:
data assimilation
Advantages
from
and
electrophysiological
benefits
our CPG
recordings
hardwareof songbird
HVC neurons
Graphite planes
HOPG – highly oriented
pyrolytic grahite
PC – pyrolytic carbon obtained
by annealing AC at 1100C
Amorphous
AC – amorphous carbon
The NDR region extends up to
the break down voltage.
Littlejohn et al. Adv. Mat 23, 2815 (2011)
A wide NDR region is observed in sp2 hybridized carbon allotropes
Amorphous carbon composites show no NDR.
The NDR is determined by the crystallographic structure of carbon
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
Repeatabilility / Joule heating
Repeatability of the I-V curve over 6 bias cycles
Noise in the NDR region is ascribed to the switching
off and re-routing of percolation paths.
Small dependency on the bias sweep rate
Can the peak arise from Joule induced heating?
Hypotheses:
• Thermal expansion of silicone matrix?
• Silicone glass transition @ 230K?
Joule heating was measured and recorded in-situ
with a Pt micro-sensor embedded in the
composite.
The NDR is not a heating effect
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
Dependence on HOPG filling fraction
Percolation transport dominates the low field
conductivity:
( p pc ) 4
At higher electric field 20kV/m (HOPG), 12kV/m
(PC) a metal insulator-transition occurs in HOPG
particles tilted at a large angle with respect to the
electric field > 66.
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
Formation of electric field domains
At the peak the composite breaks into
electric field domains. Within one domain
(homogeneous field), the NDR vanishes.
Domain boundaries = nanoparticles whose
HOPG planes are tilted by >66 in the
electric field.
Domain boundaries undergo metal-insulator
transition at the peak.
Local density of states of a crystal made of 100 layers of
graphite where VAB is applied between layers 48 and 49.
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
Multilayer transport as a function of tilt angle
Anisotropic conduction in graphite:
||
3000
Tilt angle of the electric field:
angle between the current and
the graphite layers
Between entry point (A) and exit
point (B), the current skips N
interlayers:
l ||
N
tan
c
Graphite interlayer: c=0.335nm
Diameter of HOPG NPT: l=450nm
Bilayer conduction: 66<<78
Intro
Flexible NDR
Oscillators
Amplification
Theory:
Pressure imaging
Summary
Tunnelling across the graphite/silicone barrier
Graphite
insulator
Graphite
semimetal
transparent
C
Silicone
opaque
-[Si-O]-
C
2m *
T ( E , VBC ) exp 4
3
eF
( E ) 3 / 2
E 0
0
3/ 2
3/ 2
E
( 0 E ) ( E )
Tunnelling current:
e 1k BT
J
T ( E ,VBC ) F ( E , T ,VBC ,V AB ) dE
(2 ) 2 3 v F2 0
1 exp ( E eV AB / 2)
F ( E , T ,VBC ,V AB ) ln
1 exp ( E eV AB sin / 2 eVBC )
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
Formation of electric field domains
Temperature dependence of the I-V curves of a
HOPG composite
Estimation of the graphite/silicone tunnelling
barrier height by fitting the theoretical to the
experimental dependence of the peak position.
Graphite silicone barrier height: 0=165meV.
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
Active electronic properties of the composite
Composite film on acetate
r
?
Condition for oscillations:
|g|r<1
Bend
Nogaret, J. App. Polym. Sci. DOI 10.1002/APP.40169 (2014)
The composite device generates spontaneous oscillations at frequency 𝝎 =
𝟏
𝑳𝑪
−
𝒈𝟐
𝑪𝟐
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
of the graphite/silicone
withofNDI
molecules
ProofFunctionalization
of principle: data assimilation
from electrophysiological
Running title interface
recordings
songbird
HVC neurons
Naphthalene diimide
aromatic core + appendages
Silane appendages
covalently bond to
the silicone chains
Van der Waals bonding
of aromatic cores
NDI-Si
Graphite
50% nominal
coverage of
HOPG surface
NDI-Ph
NDI-Hex
SEM image
of composite
I-V curves of
composites
functionalized
with 3 types of
NDI molecules.
77K
Littlejohn et al, Adv. Func. Mat. 23, 5398 (2013)
NDI-Si functionalization improves the properties of the graphite-silicone interface
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
Electromechanical
properties
Proof of principle: data assimilation
Device
from
processing
electrophysiological
expertise
atrecordings
Bath
of songbird HVC neurons
I-V curves under strain
Bend radius, Strain
Spontaneous oscillations
Cavity mode
Intrinsic mode
Voltage dependence of
power spectral density
Strain dependence of
emission spectrum
Bimodal emission
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
Strain dependence of power spectral density at 77K
L=47mH
rs130
Intrinsic mode
r
Cavity mode
84Hz / %strain
Bimodal spectrum with (i) a LC cavity mode with strain dependent bandwidth [ 0 - r ]; (ii) an intrinsic
mode whose frequency always increases with pressure.
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
Cut-off frequency of driven oscillations
Oscillator
Strained bilayer
Strain dependence of neg. diff. cond. g
|g| decreases with
increasing strain
NDR oscillators have a cut-off frequency which is the maximum frequency at which the oscillator
stops generating a.c. power and starts absorbing power. The impedance of the oscillator vanishes:
Z rs
Re(Z)=0 cut-off capacitance
Im(Z)=0 cut-off frequency
g
C
i
L
2
2 2
g 2 C 2 2
g
C
Cr
r g
C
gL
rs
1
1
grs
rs
r
L
r
1
1 s
grs
L
Cut –off frequency
1
grs
Increases when g
decreases i.e.
strain increases.
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
Reproducibility
under
repeated
mechanical
and
cycling
Proof of principle:
Computer
data assimilation
assisted
design
from
electrophysiological
ofRunning
printed
circuit
title board
recordings
of thethermal
demonstrator
of songbird
HVC neurons
Before setting a new
strain value, the
composite is cycled to
room temperature where
it is deformed
The composite is then
cooled down to 77K to
probe the strain
dependence of its power
spectrum.
The I-Vs are reproducible.
The I-V curves show good resilience after temperature is repeatedly cycled between 77K and 300K
and the curvature radius decreased and increased.
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
NDR amplifier
based
on ahardware
lambda
circuit
Proof of principle: data assimilation
State
from
of electrophysiological
the
art in CPG
recordings
of songbird HVC neurons
NDR region
of -circuit
AV
1
1 gr
1
gR
L
Gain
AV
1
1 gr
at freq.
1
LC
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
Flexible NDR amplifier
Flexible amplifier
Composite films amplify voltages
with voltage gain variable from 1-5.
but: signal distortion (nonlinearities).
Time series data of the input
and output voltages of a NDR
amplifier incorporating a thin
composite films as the active
element.
Varying the voltage gain of a
composite amplifier:
AV
1
1 gr
L=47mH, C=680nF, RL=1M, g0.8mS
Intro
Flexible NDR
Oscillators
Amplification
Pressure sensing:
Pressure imaging
Summary
Analogy with mechanoreceptors in the human skin
Composite sensors
encode strain into
the frequency of
voltage oscillations
(mode B: intrinsic)
f
2.1 mHz/Pa
P E1
84Hz/ %strain
Pressure sensitivity:
1s integration window
f=1 Hz
P=475 Pa
Mechanoreceptors
(Merkel cells) convert
pressure into the
frequency of voltage
oscillations.
Maricich et al., Science
324, 1580 (2009).
Biomimetic strain sensor of high sensitivity and immunity to noise that mimics Merkel cells
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
8x8 Pixel binary array demonstrator
Composite array
pitch: 0.1 in
Sensor head
symbol read by the sensor
Early demonstrator detecting binary pressure input. Pixel scan is sequential (video).
Next step multilevel pressure imaging. Large array. Parallel pixel scan.
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
64 x 64 pixel colour pressure sensor array
The read-out electronics scans the array an entire row at a time i.e. 64 sensors in parallel
Scan frequency = 3kHz. Pixel analogue data are output via a DAQ card to a PC.
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging results
Pressure imaging
Summary
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
Summary of achievements
We have demonstrated negative differential resistance in HOPG-silicone
We have identified the semimetal-insulator transition in HOPG nanoparticles as
the most likely mechanism for the NDR.
We have made the first flexible oscillator and flexible amplifier in HOPG-silicone
Demonstration that HOPG-silicone is an electronically active material
We have studied HOPG-silicone thin film under bilayer strain and demonstrated
that negative differential conductance decreases with strain.
The power spectral density of oscillators has a bimodal power spectrum
consisting of oscillations driven by the cavity (A) and intrinsic oscillations (B)
both of which only occur in the NDR region.
The frequency of mode B increases with strain and we have used this property
to make pressure sensor arrays mimicking the human skin.
Range: 50Pa 500kPa
Intro
Flexible NDR
Oscillators
Amplification
Pressure imaging
Summary
Future challenges
Raise the operating temperature of NDR devices from 200K to 300K by controlling
the thermoactivated current.
Chemical functionalization with NDI (PDI) molecules seems a promising route
Demonstrate I-V curves with multiple peaks to obtain multistate memories.
Capasso et al., IEEE Trans. Elect. Dev. 36, 2065 (1989)
Three state memory
Background
Binary array
Pressure imaging
Calibration
Conclusions
Calibration of pressure sensor in ultrasonic bath
Detection threshold : 50Pa
Relaxation time:
5s
Background
Binary array
Pressure imaging
Calibration
Conclusions
Formation of electric field domains
Background
Binary array
Pressure imaging
Calibration
Conclusions
Temperature dependence of composites with different filling fraction