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ScienceWeek
MATERIALS SCIENCE: ON PLASTIC LASERS
The following points are made by Ifor D. W. Samuel (Nature 2004 429:709):
1) Nearly all plastics are electrical insulators, but one class of plastics is different. These are conjugated polymers, whose discovery was celebrated with the award of the 2000 Nobel Prize in Chemistry to Alan Heeger, Alan G. MacDiarmid, and Hideki Shirakawa. Conjugated polymers differ from other polymers in having a backbone of alternating single and double bonds, and this difference in structure makes them semiconductors. Conjugated polymers are therefore interesting materials, with their unique combination of semiconducting properties and scope for simple fabrication and shaping. Lasers are a promising application, and Reufer et al(1) have reported fresh progress towards the creation of low-cost polymer lasers.
2) A major breakthrough in the development of semiconducting polymers was the discovery that when a voltage is applied to a thin layer of one of these materials, light can be emitted(2). Light-emitting diodes using this effect are now the basis of a modern flat-panel display technology. Other polymer optoelectronic devices have followed, including polymer solar cells, optical amplifiers, and lasers, although further development is necessary before they reach the market. Polymer lasers are attractive as light sources for several reasons. Polymers can emit light across the visible spectrum, and so wavelengths can be generated that are not readily available from other lasers. The polymers have broad spectra, meaning that a polymer laser can be tuned over a range of wavelengths. Polymer lasers should also be simple to make, can be flexible, and should be suitable for a wide range of applications, from plastic optical circuits to biological screening and assays.
3) Lasers consist of two key components: the gain medium and a resonator. The gain medium is a material in which light is amplified. In most lasers, this would be a crystalline inorganic material, such as ruby or gallium arsenide (the latter is commonly used in CD players and laser pointers). Light passes backwards and forwards through the gain medium, thanks to the feedback generated by the resonator. In plastic lasers, a semiconducting polymer is used as the gain medium(3-5). Light can be passed backwards and forwards through the polymer with mirrors(3,4), but in most polymer-laser work, including that of Reufer et al(1), a corrugated gain medium is used instead. The corrugation acts as a diffraction grating that diffracts light travelling in one direction back in the opposite direction, thereby creating feedback and enabling lasing to occur. As there are no mirrors in these lasers, they are compact, robust and exceptionally easy to align.
4) A laser needs energy in order to operate. This can be supplied in two ways: either optically (from flash-lamps or another laser) or electrically. There is a minimum energy, the threshold, required for operation. Above threshold, the gain (or amplification) exceeds all the losses in the device, and lasing begins. At present, all polymer lasers are optically excited, or "pumped". However, light emission from semiconducting polymers can be induced electrically, and an electrically pumped laser would be much more convenient.
References (abridged):
1. Reufer, M. et al. Appl. Phys. Lett. 84, 3262-3264 (2004)
2. Burroughes, J. H. et al. Nature 347, 539-541 (1990)
3. Moses, D. Appl. Phys. Lett. 60, 3215-3216 (1992)
4. Tessler, N. Adv. Mater. 11, 363-370 (1999)
5. McGehee, M. D. & Heeger, A. J. Adv. Mater. 12, 1655-1668 (2000)
Nature http://www.nature.com/nature
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Related Material:
MICROFABRICATING CONJUGATED POLYMER ACTUATORS
The following points are made by W. Edwin et al (Science 2000 290:1523):
1) The miniaturization of electronic and optical devices has fueled the information technology revolution. During the past decade, a similar miniaturization has been going on for sensors and actuators for mechanical, chemical, and biological applications. The integrated gas chromatograph was an early example (1); today, an integrated analysis system for sample handling for biological characterization has recently been developed (2).
2) Microstructures promise to be of great importance for the coming biotechnology revolution. There is currently a tremendous increase in both academic and corporate research on micromachined laboratories-on-a-chip, or micro-total analysis systems (3). These devices will find applications in areas such as genomic and proteomic studies, which will require extensive parallelism to allow many small simultaneous experiments. The integration of multiple experiments on a single carrier requires a miniature format. To minimize the chance of cross contamination when handling biological samples, a single-use device is preferred. Therefore, these devices should be disposable and thus be produced with inexpensive materials and patterning techniques. Polymers might be an option.
3) Polymers can be patterned by inexpensive methods such as hot embossing and imprinting and therefore make attractive carrier materials. Imprint methods allow submicrometer patterning with dimensions smaller than 100 nm, as has been demonstrated with hard (4) and soft (5) imprint materials. Polymers can also deliver active functions. Polymer surfaces can be chemically modified in a variety of ways, and this property is important in microstructures, which have a high surface-to-volume ratio. For example, surface-bound processes may be used to alter biomolecular function. Some polymers even allow the formation of electronic devices. Field effect transistors with useful carrier mobility have been made. Because thin polymer films may be easily prepared by spin coating, they can be integrated into functional systems. This capability may be important in the development of inexpensive, disposable chemical detectors for genomics and proteomics.
4) In summary: Conjugated polymer actuators can be operated in aqueous media, which makes them attractive for laboratories-on-a-chip and applications under physiological conditions. One of the most stable conjugated polymers under these conditions is polypyrrole, which can be patterned by means of standard photolithography. Polypyrrole-gold bilayer actuators that bend out of the plane of the wafer have been microfabricated in the laboratory of the authors. These can be used to move and position other microcomponents. The authors review the current status of these microactuators, outlining the methods used to fabricate them, and describe the devices that have been demonstrated as well as some potential future applications.
References (abridged):
1. S. C. Terry, J. H. Jerman, J. B. Angell, IEEE Trans. Electron. Devices 26, 1880
2. S. Ekstrom, et al., Anal. Chem. 72, 286 (2000)
3. A. v. d. Berg, W. Olthuis, P. Bergveld, Eds., Micro Total Analysis Systems 2000 (Kluwer, Dordrecht, Netherlands, 2000)
4. S. Y. Chou, P. R. Krauss, P. J. Renstrom, Appl. Phys. Lett. 67, 3114 (1995)
5. Y. Xia and G. M. Whitesides, Angew. Chem. Int. Ed. 37, 550 (1998)
Science http://www.sciencemag.org
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Related Material:
ON CHARGE CARRIERS IN CONJUGATED POLYMERS
Notes by ScienceWeek
An "exciton" is a quasiparticle, a combination of an electron and a positive hole, the exciton free to move through a nonmetallic crystal as a unit. Although it transports energy, the exciton has no net electrical charge. When an electron in an exciton recombines with a hole, the original atom is restored and the exciton vanishes. When this occurs, the energy of the exciton may be converted into light, or the energy may be transferred to an electron of a neighboring atom with the production of a new translocatable exciton.
The term "polaron" also refers to a quasiparticle, an electron moving through constituent atoms in a solid, with the electron causing neighboring positive charges to shift toward it and neighboring negative charges to shift away. Thus, a polaron is essentially a propagated polarization. In general, a polaron behaves as a negatively charged particle with a mass greater than that of an isolated electron, the increased mass due to the energy of interaction with the surrounding atoms of the solid. Polaron phenomena are most pronounced in ionic solids.
The following points are made by J.G. Mueller et al (Phys. Rev. Lett. 2002 88:147401):
1) The transition from a neutral excited state to a pair of charge carriers and the reverse process determine the operation of organic photodetectors and organic light emitting diodes. In the field of conjugated polymers, the interplay between emissive and nonemissive neutral states and charged quasiparticles has been a subject of intense research during the past few years (1,2). Apart from singlet and triplet intrachain excitons, pairs of oppositely charged excitations have been invoked to explain the experimental observations. Among those are spatially indirect excitons (3), intrachain and interchain polaron pairs (4), as well as geminate pairs (5). Such excitations are expected to be particularly important in the presence of a strong electric field lowering the energy of states with charge transfer character.
2) Although experiments have clearly proven the existence of such intermediate states, not much is known about the yield for the formation of such states, their dynamics and mutual interplay with singlet intrachain excitons, and their role in photocurrent generation. Recently, Arkhipov et al (1999) discussed the idea that ultrafast formation of intrachain polaron pairs due to hot exciton dissociation during vibrational relaxation is the main source for intrinsic photocarrier generation in conjugated polymers.
3) The authors report photocurrent experiments using two femtosecond laser pulses on a photodiode with a ladder-type conjugated polymer as the active layer. With a photon energy of 3.1 eV, the first pulse excites singlet excitons. A time-delayed second pulse with a photon energy of 2.49 eV leads to a decrease of the photocurrent by exciton depletion due to stimulated emission. S(sub1) excitons dissociated during their entire lifetime are identified as the only relevant channel for charge carrier generation. Intrachain polaron pairs are also formed on an ultrafast time scale with a yield of approximately 10 percent. They can be efficiently dissociated by reexcitation with photons with an energy of 1.9 eV.
References (abridged):
1. N. S. Sariciftci (ed.) Primary Photoexcitations in Conjugated Polymers: Molecular Exciton versus Semiconductor Band Model (World Scientific, Singapore, 1997).
2. M. Wohlgenannt et al., Phys. Rev. Lett. 82, 3344 (1999).
3. M. Yan et al., Phys. Rev. Lett. 72, 1104 (1994).
4. E. L. Frankevich et al., Phys. Rev. B 46, 9320 (1992).
5. U. Albrecht and H. Boessler, Chem. Phys. Lett. 235, 389 (1995).
Phys. Rev. Lett. http://prl.aps.org
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