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Quantum Electronic Devices Group Superconductivity in Metal-mixed Ion- Implanted Polymer Films Adam Micolich 1, Eric Tavenner 2, Ben Powell 2, Alex Hamilton.

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Presentation on theme: "Quantum Electronic Devices Group Superconductivity in Metal-mixed Ion- Implanted Polymer Films Adam Micolich 1, Eric Tavenner 2, Ben Powell 2, Alex Hamilton."— Presentation transcript:

1 Quantum Electronic Devices Group Superconductivity in Metal-mixed Ion- Implanted Polymer Films Adam Micolich 1, Eric Tavenner 2, Ben Powell 2, Alex Hamilton 1, 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 (For more details see A.P. Micolich et al., Cond-mat ).

2 Quantum Electronic Devices Group The Dream: Electronics on Plastic Replace this... $10 billion$ with this Economics Replace this with this Flexible circuitry Ease of Production + - Chemical Versatility Replace this with this

3 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 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?

4 Quantum Electronic Devices Group Ion-Implanted Plastics: Previous Studies For more details see S.R. Forrest et al., Appl. Phys. Lett. 41, (1982). “The room temperature resistivity of the films changes by 14 orders of magnitude from its as-deposited value of  >  cm to  5  10  4  cm at ion doses of cm  2 ” Energetic ion beams (e.g. Ar +, Kr + ) modify the near surface of insulating organic polymers to create electrical conductivity.

5 Quantum Electronic Devices Group Ion-Implanted Plastics: Previous Studies For more details see J.A. Osaheni et al., Macromol. 25, (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 ~ 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.

6 Quantum Electronic Devices Group What happens if we implant metal ions? But until recently, the best we could do is create less insulating insulators! Ion Implanted Polymers For more details see E. Tavenner et al., Synth. Met. 145, (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.

7 Quantum Electronic Devices Group 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. Ion Implanted Polymers Metal 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

8 Quantum Electronic Devices Group 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.

9 Quantum Electronic Devices Group Superconductivity In contrast to previous samples, we see a metallic temperature dependence, and a sharp drop in resistance at T < 3K. Typical T c ~ 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

10 Quantum Electronic Devices Group 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.  -ET 2 I 3 /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.

11 Quantum Electronic Devices Group Well what’s going on? - Three possible models We can immediately eliminate bulk tin as a possible explanation, our T c is suppressed and our B c 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 T c ) (i.e., we’ve just re-invented the studies in quench-condensed systems)  A tin-carbon molecular eutectic (unlikely) T c for bulk tin = 3.7K T c for our material ~2.4K! B c for bulk tin = 30.5mT B c 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.

12 Quantum Electronic Devices Group 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.

13 Quantum Electronic Devices Group 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.

14 Quantum Electronic Devices Group Chemical Consequences of Implantation 9%48%40% Sn-O, Sn=O 5%<1% Sn-C 2%14%8% Sn-Sn Sn 3d 3% 9% C-O, C=O 54%35%43% C aromatic 27%<1% C graphite C 1s Bonding Type Peak 100Å Sn with implantation 200Å Sn no implantation 100Å Sn no implantation Samples Photoelectron Properties Sn-Sn 484.4eV Sn-C 486.1eV Sn IV -O 486.5eV Sn II -O 485.3eV 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.

15 Quantum Electronic Devices Group 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 R 0 ), the superconducting transition temperature should decrease towards T = 0. ~0.4nm ~7.5nm

16 Quantum Electronic Devices Group 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 T c. Furthermore, one would naively expect that higher implant dose means more disorder and hence a higher R 0, however, we observe exactly the opposite. One final key difference with the quench condensed systems…

17 Quantum Electronic Devices Group 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.

18 Quantum Electronic Devices Group What happens if you don’t implant? ~20nm Sn:Sb film on PEEK unimplanted In the unimplanted film, the T c is 3.7K again, and the transition is very sharp

19 Quantum Electronic Devices Group 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 implanted20nm Sn:Sb film on PEEK unimplanted

20 Quantum Electronic Devices Group The bumplets are interesting A ‘peak effect’ is commonly observed in layered superconductors and in granular thin-films. MoSi: Okuma et al.DyBa 2 Cu 3 O 7-x : Wang et al. Amorphous InO x : 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, (1993); S. Okuma et al., PRB 63, (2001) and PRB 58, 2816 (1998).

21 Quantum Electronic Devices Group So what do we think is going on? += Granular Tin Smaller Granules Intimately 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.

22 Quantum Electronic Devices Group How does this sit with the electrical data? Furthermore, if the grains are small enough that they undergo T c -suppression, then one could expect that the higher implant dose gives smaller grains and therefore a lower T C, 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 R o, which we see.

23 Quantum Electronic Devices Group 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 T c with such an approach.

24 Quantum Electronic Devices Group Credits  University of NSW, Australia Dr Adam Micolich (Low T measurements) A/Prof. Alex Hamilton (Low T measurements) µ University of Queensland, Australia Eric Tavenner (Fabrication, XPS/STEM) Dr Ben Powell (Superconductor Guru) Dr Paul Meredith (Project Leader/Characterisation) µ Southwest Missouri State U., U.S.A. Dr Matthew Curry (Ion Implantation) Dr Ryan Giedd (Ion Implantation) µ Helpful Discussions Ross McKenzie, James Brooks, Arzhang Ardavan, Stephen Blundell, Andrew Briggs, Brad Marston, Urban Lundin, Des McMorrow and Francis Pratt. µ Experimental Assistance Barry Wood, Brisbane Surface Analysis Centre Centre for Microscopy and Microanalysis at the University of Queensland µ Funding

25 Quantum Electronic Devices Group Heaven is……

26 Quantum Electronic Devices Group Credits  University of NSW, Australia Dr Adam Micolich (Low T measurements) A/Prof. Alex Hamilton (Low T measurements) µ University of Queensland, Australia Eric Tavenner (Fabrication, XPS/STEM) Dr Ben Powell (Superconductor Guru) Dr Paul Meredith (Project Leader/Characterisation) µ Southwest Missouri State U., U.S.A. Dr Matthew Curry (Ion Implantation) Dr Ryan Giedd (Ion Implantation) µ Helpful Discussions Ross McKenzie, James Brooks, Arzhang Ardavan, Stephen Blundell, Andrew Briggs, Brad Marston, Urban Lundin, Des McMorrow and Francis Pratt. µ Experimental Assistance Barry Wood, Brisbane Surface Analysis Centre Centre for Microscopy and Microanalysis at the University of Queensland µ Funding

27 Quantum Electronic Devices Group  Learn what we can really do with this system (increase T c, 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 Polymers Metal Mixed ?

28 Quantum Electronic Devices Group Repeatability and Reproducibility (2) 002 Repeatability A01A09

29 Quantum Electronic Devices Group Hall Measurements For more details see V.C. Long et al., J. Appl. Phys. 80, (1996). Typical range of n was ~10 13 cm -2 (A08) to ~10 21 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.

30 Quantum Electronic Devices Group Response to a magnetic field The samples show a critical field B c that falls linearly with increasing temperature, typical for a type II thin film superconductor. One notable feature is the noise as T approaches T c. We observe critical magnetic fields B c is as high as 500 mT.

31 Quantum Electronic Devices Group Well what’s going on? - Three possible models We can immediately eliminate bulk tin as a possible explanation, our T c is suppressed and our B c 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 T c )  A tin-carbon molecular eutectic (unlikely) T c for bulk tin = 3.7K T c for our material ~2.4K! B c for bulk tin = 30.5mT B c 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.

32 Quantum Electronic Devices Group Some key properties The critical temperature T c ~ 2.4K, but ranges from around 1.9K to 2.7K (in the samples so far). The critical magnetic field B c 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 I c 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.

33 Quantum Electronic Devices Group 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

34 Quantum Electronic Devices Group 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 Granules Energetic Ions

35 Quantum Electronic Devices Group 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 : First ‘organic electronics’ appear on the market as flexible displays.

36 Quantum Electronic Devices Group Plastic Electronics are already out there…

37 Quantum Electronic Devices Group 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.


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