Superconductivity in Metal-mixed IonImplanted Polymer Films
(For more details see A.P. Micolich et al., Cond-mat 0509278).
Adam Micolich 1, Eric Tavenner 2, Ben Powell 2,
Alex Hamilton1, Matt Curry 3,Ryan Giedd 3, and Paul Meredith 2.
1 School
of Physics, University of New South Wales, Sydney 2052, Australia.
2 Physics Department, University of Queensland, Brisbane 4072, Australia
3 Center for Applied Science and Engineering, Southwest Missouri State
Quantum Electronic
University, Springfield MO 65804. Devices Group
The Dream: Electronics on Plastic
Replace this . . .
. . . with this
Replace this . . .
. . . with this
Flexible circuitry
Replace this . . .
. . . with this
$10 billion
$100
Economics
+
Ease of Production
Chemical Versatility
Quantum Electronic
Devices Group
An Alternative Approach
You could deposit
conducting polymer
inks?
+
Many conducting polymers are
air/moisture-sensitive, they don't stick
well, and are often quite expensive.
Instead, could you use ionimplantation to modify the
conductivity of plastic, just like
we do with Silicon already!
And so develop a cheap and simple way
to create conducting plastics?
Quantum Electronic
Devices Group
Ion-Implanted Plastics: Previous Studies
Energetic ion beams (e.g. Ar+, Kr+) modify the near surface of insulating organic polymers to create
electrical conductivity.
“The room temperature resistivity of the films changes by 14 orders of magnitude from its as-deposited value of
r > 1010 Wcm to r  5  10-4 Wcm at ion doses of 1017 cm-2”
For more details see S.R. Forrest et al., Appl. Phys. Lett. 41, 708-710 (1982).
Quantum Electronic
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Ion-Implanted Plastics: Previous Studies
Organic polymer “de-polymerises” under the ion beam, volatiles (O, H, N) are lost, and recrosslinking creates carbon rich clusters.
“XPS data of films revealed significant reduction in the heteroatoms and increased carbon content after
implantation … The room temperature conductivity of these implanted polymers, typically ~80-200 S/cm, is
significantly higher than that obtained to date by conventional doping techniques.”
For more details see J.A. Osaheni et al., Macromol. 25, 5828-5835 (1992).
Quantum Electronic
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What happens if we implant metal ions?
All of the inert ion implanted films are strongly insulating, in an attempt to improve the conductivity
a shift was made to using metallic ions as the implant species.
Ion Implanted Polymers
Sample
Resistivity (Wcm)
Conductivity (S/cm)
PEEK
4.90E+16
2.04E-17
N+ Ion Implanted PEEK
1.02E+06
9.80E-07
Sn+ Ion Implanted PEEK
11.25
0.09
But until recently, the best we could do is create less insulating insulators!
For more details see E. Tavenner et al., Synth. Met. 145, 183-190 (2004).
Quantum Electronic
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We need more metal in the surface…
Implanting with metal beams can’t get enough metal into the plastic to confer metallic conductivity.
There is a limiting dose because the beam starts to sputter the deposited ions away.
However, you can instead deposit a thin layer of Sn:Sb (95:5) on the surface (as little as ~10nm) and
‘push’ this into the PEEK using a nitrogen or argon ion-beam.
Ion Implanted Polymers
Metal Mixed
:Sb
:Sb
Sample
Resistivity (Wcm)
Conductivity (S/cm)
PEEK
4.90E+16
2.04E-17
N+ Ion Implanted PEEK
1.02E+06
9.80E-07
Sn+ Ion Implanted PEEK
11.25
0.09
Sn 'metal mixed' into PEEK
2.92E-05
34246.58
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Devices Group
Experimental Methods
After Sn:Sb evaporation, implantation is done using an IBM Taconic Implanter at 50kV.
The samples are prepared for electrical measurements by evaporating Ti/Au contacts in the corners
and using InAg solder to attach Cu wires.
Samples were measured on an Oxford Instruments VTI system capable of temperatures between
200K and 1.2K, and magnetic fields up to 10T.
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Superconductivity
In contrast to previous samples, we see a metallic temperature dependence, and a sharp drop in
resistance at T < 3K.
Superconductivity confirmed by both two terminal (left) and four terminal (right) measurements.
RRR < 1.2 (ie. highly
disordered metal)
Typical Tc ~ 1.9 – 2.7K
For more details see A.P. Micolich et al., Cond-mat 0509278.
Quantum Electronic
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Superconductor/Polymer Composites
Superconductor/polymer blends (e.g. b-ET2I3/Polycarbonate) studied previously have
shown an incomplete superconducting transition and a partial Meissner effect.
Laukhina et al.
“At some conditions of the thermal treatment an accelerated
decrease of resistivity is observed below 7K, vanishing with
application of magnetic field, thus giving the first evidence of
the (incomplete) superconducting transition in polymeric
organic composite material.”
For more details see E.E. Laukhina et al., Synth.
Met. 70, 797 (1995) and A. Tracz et al., Synth. Met.
120, 849 (2001).
Our material is that it is the first superconductor/polymer composite to show a zero
resistance electrical state. (as far as we know)
Quantum Electronic
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Well what’s going on? - Three possible models
•
•
We can immediately eliminate bulk tin as a possible explanation, our Tc is suppressed and our Bc
is enhanced compared to the bulk tin values.
Tc for bulk tin = 3.7K
Bc for bulk tin = 30.5mT
Tc for our material ~2.4K!
Bc for our material as high as 500mT!
This leaves only three possible models (that we know of) for the origin of superconductivity in
this material.
 A continuous thin film of tin (has to be thin enough to suppress Tc)
(i.e., we’ve just re-invented the studies in quench-condensed systems)
 A layer of granular tin mixed into a partially conducting hydrocarbon matrix
 A tin-carbon molecular eutectic (unlikely)
•
Let’s look at some evidence to support/eliminate some of these models.
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Quench-Condensed Metal Studies
Metals (typically Bi, Sn, Pb, etc.) are deposited by MBE onto a flat solid substrate held at 4K.
Low temperature deposition under UHV conditions is essential to producing these ~10-100Å
thick films and keeping them stable enough to study.
For a brief review see A.M. Goldman and N. Markovic, Physics Today Nov 1998, p. 39.
Quantum Electronic
Devices Group
Metal Mixing – Does it really work that way?
Cross-sectional Scanning Tunnelling Electron Microscopy (STEM) and Energy Dispersive X-ray
Analysis (EDX) shows an implant mixed region that extends ~75nm into the PEEK sub-surface.
This is over 7 times the thickness of the original 10nm Sn film we deposited.
The implantation encapsulates the tin and thereby significantly enhances its adhesion to the
plastic, which retains its native mechanical properties.
Measurements repeated over a period of seven months with little change beyond a slight (< 10%)
increase in the normal resistance. This is despite storage in a plastic box under ambient
conditions.
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Chemical Consequences of Implantation
X-ray Photoelectron Spectroscopy shows dramatic changes in the composition of the implant
region.
SnII-O Sn-Sn
SnIV-O Sn-C
485.3eV
484.4eV
486.5eV 486.1eV
Samples
Photoelectron
Properties
Peak
C 1s
Sn
3d
Bonding
Type
100Å Sn
no
implantation
200Å Sn
no
implantation
100Å Sn
with
implantation
C graphite
<1%
<1%
27%
C aromatic
43%
35%
54%
C-O, C=O
9%
3%
3%
Sn-Sn
8%
14%
2%
Sn-C
<1%
<1%
5%
Sn-O, Sn=O
40%
48%
9%
Implantation does three key things:
1: Reduce the Sn-Sn and Sn-O bonds by a factor of ~5 =>
breaks up the metal.
2: Increases the Sn-C bonds by a factor of ~5
=>
binds the tin to the plastic.
3: Massively increases the graphitic carbon content
=>
same effect as with no metal.
Quantum Electronic
Devices Group
Quench-Condensed Studies – Electrical Behaviour
~0.4nm
~7.5nm
The quench-condensed studies of most metals (including Sn) show a thickness-controlled
superconductor-insulator transition (SIT).
Of particular note, as you increase the film disorder (i.e., increase the normal film resistance R0), the
superconducting transition temperature should decrease towards T = 0.
For a brief review see A.M. Goldman and N. Markovic, Physics Today Nov 1998, p. 39.
Quantum Electronic
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How does our data compare to this?
We find behaviour that is very different to the QC studies – The sample with the higher normal
resistance actually has the higher critical temperature Tc.
Furthermore, one would naively expect that higher implant dose means more disorder and hence a
higher R0, however, we observe exactly the opposite.
One final key difference with the quench condensed systems…
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The Antimony fraction is essential in our samples
Pure Sn
95% Sn : 5% Sb
If we use pure Sn, samples with thicknesses up to 40 nm are strongly insulating, whether they are
implanted or not.
Sb is commonly used as an impurity in Sn solders to inhibit the transition from the metallic white
allotrope to the insulating grey allotrope.
However, whether Sb plays any role beyond this in the post-implant structure is not yet clear.
Quantum Electronic
Devices Group
What happens if you don’t implant?
~20nm Sn:Sb film on PEEK unimplanted
In the unimplanted film, the Tc is 3.7K again, and the transition is very sharp
Quantum Electronic
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What happens if you don’t implant?
10nm Sn:Sb film on PEEK implanted
20nm Sn:Sb film on PEEK unimplanted
The field dependence for the unimplanted films shows something quite interesting,
there are ‘bumplets’ on the high-field side of the field-induced superconducting transition!
Quantum Electronic
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The bumplets are interesting
A ‘peak effect’ is commonly observed in layered superconductors and in granular thin-films.
Amorphous InOx: Paalanen et al.
DyBa2Cu3O7-x: Wang et al.
MoSi: Okuma et al.
“An anomalous peak in the perpendicular MR has been also observed in granular films, whose origin is related to
destruction of local superconductivity within each grain.” (Okuma et al.)
For more details see M.A. Paalanen et al., PRL 69, 1604 (1992); T. Wang et al., PRB 47, 11619 (1993);
S. Okuma et al., PRB 63, 054523 (2001) and PRB 58, 2816 (1998).
Quantum Electronic
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So what do we think is going on?
+
Granular Tin
=
Energetic Ions
Smaller Granules
Intimately Mixed into
the Substrate
•
We actually start out with an granular/amorphous alloy coating on the PEEK in the evaporation
step, similar to that found with other materials on other substrates.
•
The incident energetic ions then lead to smaller granules intimately mixed into the sub-surface of
the PEEK.
Quantum Electronic
Devices Group
How does this sit with the electrical data?
•
For the higher implant dose, one could expect that it has smaller grains with a smaller inter-grain
separation, and since the inter-grain hopping scales with the grain separation, this should have
the lower Ro, which we see.
•
Furthermore, if the grains are small enough that they undergo Tc-suppression, then one could
expect that the higher implant dose gives smaller grains and therefore a lower TC, which we also
see.
Quantum Electronic
Devices Group
So what next? – Short term
A lot more samples and a lot more measurements. We want to:
 Study a more comprehensive range of metal thicknesses to understand how pre-implant
metal thickness determines the material properties.
 Perform further materials characterisation studies (e.g., small-angle neutron scattering) to
better establish the structural and chemical details of the implant region.
 Explore whether other metals are suitable for this technique, and whether we can
raise Tc with such an approach.
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Devices Group
Credits
 University of NSW, Australia
Dr Adam Micolich (Low T measurements)
 Funding
A/Prof. Alex Hamilton (Low T measurements)
 University of Queensland, Australia
Eric Tavenner (Fabrication, XPS/STEM)
Dr Ben Powell (Superconductor Guru)
Dr Paul Meredith (Project Leader/Characterisation)
 Southwest Missouri State U., U.S.A.
Dr Matthew Curry (Ion Implantation)
Dr Ryan Giedd (Ion Implantation)
 Helpful Discussions
Ross McKenzie, James Brooks, Arzhang Ardavan, Stephen Blundell,
Andrew Briggs, Brad Marston, Urban Lundin, Des McMorrow and Francis Pratt.
 Experimental Assistance
Barry Wood, Brisbane Surface Analysis Centre
Centre for Microscopy and Microanalysis at the University of Queensland
Quantum Electronic
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Heaven is……
Quantum Electronic
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Credits
 University of NSW, Australia
Dr Adam Micolich (Low T measurements)
 Funding
A/Prof. Alex Hamilton (Low T measurements)
 University of Queensland, Australia
Eric Tavenner (Fabrication, XPS/STEM)
Dr Ben Powell (Superconductor Guru)
Dr Paul Meredith (Project Leader/Characterisation)
 Southwest Missouri State U., U.S.A.
Dr Matthew Curry (Ion Implantation)
Dr Ryan Giedd (Ion Implantation)
 Helpful Discussions
Ross McKenzie, James Brooks, Arzhang Ardavan, Stephen Blundell,
Andrew Briggs, Brad Marston, Urban Lundin, Des McMorrow and Francis Pratt.
 Experimental Assistance
Barry Wood, Brisbane Surface Analysis Centre
Centre for Microscopy and Microanalysis at the University of Queensland
Quantum Electronic
Devices Group
So what next? – The future
 Learn what we can really do with this system (increase Tc, minimum feature size, etc.).
 Work on creating patterned versions with the view towards making devices such as Josephson
Junctions and ultimately SQUIDs.
 Longer term, work on achieving the missing conductivity regime (i.e., a proper semiconductor
with a band-gap, etc.) – amorphous Si in PEEK?
Ion Implanted Polymers
Metal Mixed
?
Quantum Electronic
Devices Group
Repeatability and Reproducibility
Repeatability
002
001
A01
001(2)
A09
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Devices Group
Hall Measurements
The original plan at this point was to try and get Hall effect data for these samples in order to
establish the carrier type/density and mobility of the material.
For more details see V.C. Long et al.,
J. Appl. Phys. 80, 4202-4204 (1996).
Typical range of n was ~1013 cm-2 (A08)
to ~1021 cm-2 (001)
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Response to a magnetic field
We observe critical magnetic fields Bc is as high as 500 mT.
The samples show a critical field Bc that falls linearly with increasing temperature, typical for a type
II thin film superconductor. One notable feature is the noise as T approaches Tc.
Quantum Electronic
Devices Group
Well what’s going on? - Three possible models
•
•
We can immediately eliminate bulk tin as a possible explanation, our Tc is suppressed and our Bc
is enhanced compared to the bulk tin values.
Tc for bulk tin = 3.7K
Bc for bulk tin = 30.5mT
Tc for our material ~2.4K!
Bc for our material as high as 500mT!
This leaves only three possible models (that we know of) for the origin of superconductivity in
this material.
 A continuous thin film of tin (has to be thin enough to suppress Tc)
 A layer of granular tin / partially conducting hydrocarbon
 A tin-carbon molecular eutectic (unlikely)
•
Let’s look at some evidence to support/eliminate some of these models.
Quantum Electronic
Devices Group
Some key properties
•
The observed metallic and superconducting behavior is repeatable after thermal cycling to room
temperature, and reproducible (quantitatively similar) in nominally identical samples.
•
The upper bound on the residual resistance ratio (RRR) is 1.2, indicating that our material is a
highly disordered metal.
•
The critical temperature Tc ~ 2.4K, but ranges from around 1.9K to 2.7K (in the samples so far).
•
The critical magnetic field Bc is as high as 500 mT.
•
The critical current Ic is of order 1 mA, with superconductivity occasionally observed at currents
as high as 10 mA.
•
We find that the metal-mixed layer does not delaminate even after several cryogenic cycles and
the implanted material shows significant durability.
•
We have repeated our measurements over a period of seven months with little change or
degradation of the electronic properties beyond a slight (< 10%) increase in the normal
resistance over this period, despite simply storing these samples in a plastic box under ambient
conditions.
Quantum Electronic
Devices Group
Summary and Conclusion

Ion-implantation can be used to create cheap conducting polymers – metal mixing can produce
plastic materials with metallic and superconducting properties.

These implanted plastics retain the native mechanical properties of the bulk material – they are
flexible and robust.
Superconductivity in the Metal-mixed Systems

While the continuous thin film is the simplest and most logical conclusion, our combined
evidence (structural, chemical and electrical studies) suggests that this is not the case.

Instead, we propose that our plastic superconductor is either a mixed (tin + hydrocarbon)
granular system (with very small granules potentially) or a molecular tin-carbon eutectic. Future
studies will be aimed at addressing these possibilities further.
Quantum Electronic
Devices Group
So what does all this tell us?
200Å film on PEEK unimplanted
100Å film on PEEK implanted
implantation
What this suggests is, that we start out with a granular system, the ion beam smashes up the
granules and mixes them into the PEEK, giving us our superconductor/polymer composite.
+
Granular Tin
=
Energetic Ions
Smaller Granules
Intuitively this makes some sense. Also, the intimate mixing might explain why layers too thin to
conduct before implantation start to conduct after implantation.
Quantum Electronic
Devices Group
Conducting Polymers
•
Early 1970’s: Mistake in Skirakawa’s lab
leads to accidental discovery of silver
looking polymer (polyacetylene)
•
Late 1970’s: Collaboration between
Heeger, MacDiarmid and Shirakawa lead
to 10 million-fold increase in conductivity
of polyacetylene.
•
2000: Heeger, MacDiarmid and Shirakawa
win Nobel Prize in Chemistry
•
2000+: First ‘organic electronics’ appear
on the market as flexible displays.
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Plastic Electronics are already out there…
Quantum Electronic
Devices Group
The encapsulation gives some advantages
•
Metallic and superconducting behavior is repeatable after thermal cycling, and relatively
reproducible in nominally identical samples.
•
The metal-mixed layer does not delaminate even after several cryogenic cycles.
•
Measurements repeated over a period of seven months with little change beyond a slight (< 10%)
increase in the normal resistance. This is despite storage in a plastic box under ambient
conditions.
… sure beats …
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