Objectives
- To improve our fundamental understanding of electrical conductivity produced in polymers by ion implantation
- To develop high technology devices based upon ion implanted conductive polymers
Introduction
Conductive polymers are a new group of materials that are finding applications in all type of electronic devices. Prototypical flat displays, infrared sensors, strain gauges and memory devices have already been produced. Polymers were discovered to have semi-conductive properties late in the last century (I always wanted to say that), but it hasn't been until very recently that their usefulness are being explored fully. Conductive polymers are a group of polymers that, of course, conduct electricity. What we are studying, ion implanted polymers, is a subclass of conductive polymers.
Why Do We Need Electrically Conducting Polymers?
Polymers have advantages over traditional inorganic conductors and semi-conductors:
- easier to process
- cheaper and safer to fabricate
- more readily available
- better performance in some applications
Main Research Trends
Inherently Conductive Polymers
The main focus of current research into conductive polymers is geared towards
inherently conductive and semi-conductive species. The methods used to change their characteristics
are the careful control of their growth, and in melt and/or post melt doping.
Now knowing this, you may ask...
There are already conductive polymers out there, why investigate ion
implanted polymers?
Good question. With ion implantation, we can change key properties of many common, insulating polymers. These include changing electrical properties, increasing resistance to solvents, increasing it’s adhesiveness, or changing basic elemental makeup. Ion implanted polymers can be a bridging technology between current device fabrication techniques and developing polymer technology.
Polymers Used in This Research
The aims of our research are to take inherently insulating polymers, change their electrical conductivity through ion implantation, and develop a model explaining how the implant parameters affect the conductivity mechanism (also known as the charge transport mechanism).
The Implantation Process
Ion implantation occurs within a large instrument called, you guessed it, an ion implanter. During ion implantation, a beam of high energy ions are fired at a sample. These ions interact with the sample, changing its properties. Ion implanters are already widely used in the semi-conductor industry for implanting silicon.
Ion Implanter in Use at CASE
Upon collision of the ion with the sample there are several processes that can occur. One is the substitution of a sample atom with an implanted one. Another is the creation of a cascade damage region (a region where the order is changed). Sputtering of the surface can also occur (where the topmost layers are removed). Finally, if there is another material on top of the sample (i.e. surface contamination, oxide layer, etc.), then blending of the two materials occurs.
Modifying and Understanding Electrical Conductivity
There is no clear picture as to how ion implantation modifies electrical conductivity. We plan to establish the conditions necessary for “tuning” the electrical conductivity of common polymers, and, additionally, establish a mechanism for explaining the conductivity modifications. Many of the facilities required for this research are available at the University of Queensland (e.g. Centre for Microscopy and Microanalysis, Centre for Magnetic Resonance, Brisbane Surface Analysis Facility, etc.) and at the Missouri State University (e.g. Ion Implantation Laboratory, Materials Characterisation Laboratory, Molecular Beam Epitaxy Laboratory, etc.)
Device Fabrication
The ultimate goal of this research is to produce viable devices. In particular, we feel that ion implanted polymers could make a substantial contribution to the development of integrated soft electronics and by providing an inorganic/organic bridging technology. In addition, ion implanted polymers could find applications in:
- sensors (e.g. infrared-already in development at SMSU, pressure, strain gauges, etc.)
- optoelectronics (e.g. photodiodes, LEDs, solid state lasers, etc.)
- photovoltaics (solar cells)
- integrated circuit elements (e.g. interconnects, capacitors, resistive tracks, etc.)
Research Plan
- Preparation of Materials - prepare implanted polymers with different ion species and implant doses
- Characterisation of Materials - this will discover the composition and properties of the polymers
- Structural
- Chemical
- Electrical
- Understanding of Transport Mechanisms - modelling of the charge transport properties based on polymer type, ion species, implant dose and implantation method
- Device Implementation - the prototyping of functional electronic devices using ion implanted polymers
Characterisation Methods
- Scanning Electron Microscopy (SEM)
- Transmission Electron Microscopy (TEM)
- Scanning Tunnelling Microscopy (STM)
- Atomic Force Microscopy (AFM)
- X-ray Photoelectron Spectroscopy (XPS)
- X-ray Diffraction (XRD)
- Rutherford Backscattering Spectroscopy (RBS)
- Elastic Recoil Detection Spectroscopy (ERDS)
- Fourier Transform Infrared Spectroscopy (FTIR)
- Ultraviolet-Visible Spectroscopy
- Four-point Probe Electroanalysis
- Hall Effect Analysis
Results Synopsis
- Resistivity of native polymer can be changed by 20 orders of magnitude depending upon ion energy, dose, species and film overlayer.
- Graphite is formed within the implanted region.
- Metal species, either as the implanted ion or as a film overlayer, form bonds with the carbon in the polymer and with the forming graphitic regions.
- Optical absorption increases as a function of dose, however apparent saturation occurs at a dose of 1017 ions/sq.cm.
- Certain samples displays superconductive properties (e.g. zero resistance at low temperatures, critical magnetic field, etc.), depending on various parameters that are not entirely clear at the moment.
Publications
Tailored conductivity in ion-implanted polyetheretherketone
E. Tavenner, P. Meredith, B. Wood, M. Curry, R. Giedd
Synthetic Metals
Vol. 145 (2004) pgs. 183-190
Ion beam modification and analysis of metal/polymer bi-layer thin films
Y.Q. Wang, M. Curry, E. Tavenner, N. Dobson, R.E. Giedd
Nuclear Instruments and Methods in Physics Research B
Vols. 219-220 (2004) pgs. 798-803
Superconductivity in metal-mixed ion-implanted polymer films
A.P. Micolich, E. Tavenner, B.J. Powell, A.R. Hamilton, M.T. Curry, R.E. Giedd and P. Meredith
Applied Physics Letters
Vol. 89 (2006) 152503
Determination of thermal and optical properties of ion implanted PEEK films by photothermal spectroscopies
J.E. de Albuquerque, E. Tavenner, M. Curry, R.E. Giedd, P. Meredith
Journal of Applied Physics
Vol. 101 (2007) 054506
Historical background of this project
The driving force for this research is for a greater understanding of a polymer based material I developed in 1999 while working at the Southwest Missouri State University (now just Missouri State University). I then developed a research plan and submitted it to the University of Queensland late in the following year, upon which I was accepted for PhD study and subsequently received two scholarships for this study, commencing in 2002. I completed my PhD late in 2006, and I am currently continuing with this investigation I started nearly a decade ago. The research plan has change little since it's initial conception. This is because there are only a limited number of characterisation methods that are readily available, it has been discovered that a complete picture cannot be gained from just a couple of methods, and often complimentarily methods are needed to gain a better understanding. As with any research, the future is determined by funding that can be acquired. Recently a small grant was obtained, and with any luck future funding will be forthcoming. After some more articles are published, I will be placing my PhD thesis in its entirety on this web site.