(for more details see a.p. micolich et al., cond-mat 0509278)

Quantum ElectronicDevices Group

Superconductivity in Metal-mixed Ion-Implanted Polymer Films

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

University, Springfield MO 65804.

(For more details see A.P. Micolich et al., Cond-mat 0509278).


The Dream: Electronics on Plastic

Replace this . . .

$10 billion $100

. . . with this

Economics

Replace this . . . . . . with this

Flexible circuitry

Ease of Production

+ -

Chemical Versatility

Replace this . . .

. . . with this


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 ion-implantation 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?


Ion-Implanted Plastics: Previous Studies

For more details see S.R. Forrest et al., Appl. Phys. Lett. 41, 708-710 (1982).

“The room temperature resistivity of the films changes by 14 orders of magnitude from its as-deposited value of > 1010

cm to 5 104 cm at ion doses of 1017 cm2”

Energetic ion beams (e.g. Ar+, Kr+) modify the near surface of insulating organic polymers to create electrical conductivity.


Ion-Implanted Plastics: Previous Studies

For more details see J.A. Osaheni et al., Macromol. 25, 5828-5835 (1992).

“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.”

Organic polymer “de-polymerises” under the ion beam, volatiles (O, H, N) are lost, and re-crosslinking creates carbon rich clusters.


What happens if we implant metal ions?

But until recently, the best we could do is create less insulating insulators!

Ion Implanted Polymers

Sample Resistivity (cm) 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

For more details see E. Tavenner et al., Synth. Met. 145, 183-190 (2004).

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.


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.

Sample Resistivity (cm) 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

Ion Implanted PolymersMetal Mixed

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.

:Sb

:Sb


Experimental Methods

Samples were measured on an Oxford Instruments VTI system capable of temperatures between 200K and 1.2K, and magnetic fields up to 10T.

The samples are prepared for electrical measurements by evaporating Ti/Au contacts in the corners and using InAg solder to attach Cu wires.

After Sn:Sb evaporation, implantation is done using an IBM Taconic Implanter at 50kV.


SuperconductivityIn contrast to previous samples, we see a metallic temperature dependence, and a sharp drop in resistance at T < 3K.

Typical Tc ~ 1.9 – 2.7K

Superconductivity confirmed by both two terminal (left) and four terminal (right) measurements.

RRR < 1.2 (ie. highly disordered metal)

For more details see A.P. Micolich et al., Cond-mat 0509278.


Superconductor/Polymer Composites

“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.”

Superconductor/polymer blends (e.g. -ET2I3/Polycarbonate) studied previously have shown an incomplete superconducting transition and a partial Meissner effect.

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)

Laukhina et al.


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.

A layer of granular tin mixed into a partially conducting hydrocarbon matrix

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 tin-carbon molecular eutectic (unlikely)

Tc for bulk tin = 3.7K

Tc for our material ~2.4K!

Bc for bulk tin = 30.5mT

Bc for our material as high as 500mT!

• Let’s look at some evidence to support/eliminate some of these models.

• This leaves only three possible models (that we know of) for the origin of superconductivity in this material.


Quench-Condensed Metal Studies

For a brief review see A.M. Goldman and N. Markovic, Physics Today Nov 1998, p. 39.

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.


Metal Mixing – Does it really work that way?

The implantation encapsulates the tin and thereby significantly enhances its adhesion to the plastic, which retains its native mechanical properties.

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.

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.


Chemical Consequences of Implantation

9%48%40%Sn-O, Sn=O

5%<1%<1%Sn-C

2%14%8%Sn-Sn

Sn 3d

3%3%9%C-O, C=O

54%35%43%C aromatic

27%<1%<1%C graphite

C 1s

BondingTypePeak

100Å Sn with

implantation

200Å Snno

implantation

100Å Snno

implantation

SamplesPhotoelectron Properties

Sn-Sn484.4eV

Sn-C486.1eV

SnIV-O486.5eV

SnII-O485.3eV

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.

X-ray Photoelectron Spectroscopy shows dramatic changes in the composition of the implant region.


Quench-Condensed Studies – Electrical Behaviour

For a brief review see A.M. Goldman and N. Markovic, Physics Today Nov 1998, p. 39.

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.

~0.4nm

~7.5nm


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…


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.


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


What happens if you don’t implant?

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!

10nm Sn:Sb film on PEEK implanted 20nm Sn:Sb film on PEEK unimplanted


The bumplets are interesting A ‘peak effect’ is commonly observed in layered superconductors and in granular thin-films.

MoSi: Okuma et al.DyBa2Cu3O7-x: Wang et al.Amorphous InOx: Paalanen 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).


So what do we think is going on?

+ =

Granular Tin Smaller GranulesIntimately Mixed into

the Substrate

Energetic Ions

• The incident energetic ions then lead to smaller granules intimately mixed into the sub-surface of the PEEK.

• 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.


How does this sit with the electrical data?

• 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.

• 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.


So what next? – Short term

A lot more samples and a lot more measurements. We want to:

Perform further materials characterisation studies (e.g., small-angle neutron scattering) to better establish the structural and chemical details of the implant region.

Study a more comprehensive range of metal thicknesses to understand how pre-implant metal thickness determines the material properties.

Explore whether other metals are suitable for this technique, and whether we can raise Tc with such an approach.


Credits University of NSW, Australia

Dr Adam Micolich (Low T measurements)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

Funding


Heaven is……


Credits University of NSW, Australia

Dr Adam Micolich (Low T measurements)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

Funding


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?

So what next? – The future

Ion Implanted PolymersMetal Mixed

?


Repeatability and Reproducibility

001 001(2)

002

Repeatability

A01 A09


Hall Measurements

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)

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.


Response to a magnetic field

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.

We observe critical magnetic fields Bc is as high as 500 mT.


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.

A layer of granular tin / partially conducting hydrocarbon

A continuous thin film of tin (has to be thin enough to suppress Tc)

A tin-carbon molecular eutectic (unlikely)

Tc for bulk tin = 3.7K

Tc for our material ~2.4K!

Bc for bulk tin = 30.5mT

Bc for our material as high as 500mT!

• Let’s look at some evidence to support/eliminate some of these models.

• This leaves only three possible models (that we know of) for the origin of superconductivity in this material.


Some key properties

• 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 upper bound on the residual resistance ratio (RRR) is 1.2, indicating that our material is a highly disordered metal.

• The critical current Ic is of order 1 mA, with superconductivity occasionally observed at currents as high as 10 mA.

• The observed metallic and superconducting behavior is repeatable after thermal cycling to room temperature, and reproducible (quantitatively similar) in nominally identical samples.

• 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.


Summary and Conclusion

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.

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


So what does all this tell us?

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.

+ =

100Å film on PEEK implanted200Å film on PEEK unimplanted

implantation

Intuitively this makes some sense. Also, the intimate mixing might explain why layers too thin to conduct before implantation start to conduct after implantation.

Granular Tin Smaller GranulesEnergetic Ions


Conducting Polymers

• Early 1970’s: Mistake in Skirakawa’s lab leads to accidental discovery of silver looking polymer (polyacetylene)

• 2000: Heeger, MacDiarmid and Shirakawa win Nobel Prize in Chemistry

• Late 1970’s: Collaboration between Heeger, MacDiarmid and Shirakawa lead to 10 million-fold increase in conductivity of polyacetylene.

• 2000+: First ‘organic electronics’ appear on the market as flexible displays.


Plastic Electronics are already out there…


The encapsulation gives some advantages

… sure beats …

• 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.

(for more details see a.p. micolich et al., cond-mat 0509278)

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