SCIENCE & TECHNOLOGY
STABLE SOLUBLE SEMICONDUCTOR Organometallic compound does not degrade when exposed to light, air, and water
A
PLATINUM-BASED
ORGANO-
metallic material synthesized by scientists in Europe offers a promising solution to the problem ofpoor environmental stability that is suffered by virtually all known semiconducting organic polymers and layered organic-inorganic hybrid materials. The material, a dimethyloctane (dmoc) compound, [Pt(NH 2 dmoc) 4 HPtCl 4 }, has a one-dimensional chain structure with a backbone of linearly arranged platinum atoms. It is based on Magnus' green salt, [Pt(NH3) 4 KPtCl 4 ], first reported in 1828 by German scientist Heinrich Gustav Magnus (1802-70). The semiconducting organic material was synthesized, characterized, and processed into devices in a collaborative effort by researchers at the Swiss Federal Institute of Technology in Zurich (ETH); the Center for Research on Plant Macromolecules and the Netherlands Organization for Scientific Research, Grenoble, France; Eindhoven University of Technology and Delft: University of Technology,
PLATINUM Compound has one-dimensional chain structure NHoR RH2N —Pt—NH 2 R RH2N/j CI
Cl
N*'C l CI ' \ X
I
NH2R
RH2N — Pt—-NH2R RH2N/j Cl X Cl /
Pt
X
: \
2+ NH9R / RH2N — Pt—NH 2 R R H ^
Cl Cl CH, I R = (CH2)2CH(CH2)3CH(CH3)2
44
C&EN
/ MARCH
1 0,
2003
the Netherlands; and the University of Cambridge [Adv. Mater., 15,125 (2003)1. "Our work addresses the old, but not always openly exposed, problem of the environmental stability of popular organic semiconductors," E T H professor of polymer technology Paul Smith tells C&EN. "Typically, these materials are manufactured and processed into devices in dryboxes, in the absence of white light, air, and/or water. Also, once incorporated in devices, elaborate packaging is often applied to increase their lifetime. "We synthesized the semiconducting platinum-based chain structures in aqueous media and processed them from simple organic solvents in air to make oriented thin films, fibers, and field-effect transistors," he continues. iCWe found that the materials can be exposed to white light and air for six months or more without significant loss of their semiconducting characteristics." THE PLATINUM MATERIALS c a n w i t h -
stand extremely harsh conditions. Even "cooking" in hot water at 80 to 90 °C for periods of 12 hours or so does not lead to significant loss of performance. "This treatment even improves the onoff switching ratios of field-effect transistors by a factor of 10 or more," Smith says. "We realize that our materials and devices do not yet exhibit outstanding semiconducting characteristics such as charge mobility However, we see this effort as a start toward a class of environmentally stable, readily processible compounds that, because of their simple chemistry could include a broad spectrum of additional functionalities." The team synthesized compounds of the type [Pt(NH 2 R) 4 HPtCl 4 l, where R is a linear or branched alkyl group, by adding K 2 [PtCl 4 ] to a solution of the selected amine compound in water. The products were extracted using a solvent such as toluene. {Pt(NH2dmoc)4HPtCl4l—agreen, crystalline material that is thermally stable to over 130 °C—is highly soluble at around 70 to 80 °C in a range of solvents, including toluene, trichloroethane, and xylene. The platinum compound can be easily re-
crystallized under ambient conditions. "This very desirable property makes it possible to readily form films, fibers by electrostatic spinning, blends with polymers, and other structures," the authors note. They used {Pt(NH 2 dmoc) 4 HPtCl 4 ] as the active semiconductor layer to produce field-effect transistors under ambient conditions in air. By using poly(tetrafluoroethylene) orientation layers, they were able to align the platinum chain structures parallel to the current transport direction exhibited by p-type transistor action and to determine the field-effect mobilities of
MAGNUS' SALT Margherita Fontana, a Ph.D. student at ETH, prepares novel derivatives of the platinum compound. the devices. They attributed the mobility to the injection of holes into a valence band based on platinum d-orbitals delocalized along the polymer axis. The team, however, was unable to obtain n-type transistor operation using the platinum compound. The paper makes two important contributions to the development of "printable" transistor technology, according to Howard E. Katz, an organic chemist at Lucent Technologies' Bell Laboratories. Katz and colleagues have used conventional organic semiconductors to fabricate thinfilm organic field-effect transistors that operate under a pool of liquid water [Lang/ ^ 18,5299 (2002)}. "The introduction of the platinum chain structure opens up more of the periodic table of elements for consideration as building blocks for solution-processible semiconductors," he says. "This will encourage broader approaches to semiconductor synthesis by both organic and inorganic chemists. "The stability of the films to water makes them candidates for use in transistor sensors aimed at detecting aqueous analytes, besides pointing to a possible solution of the water degradation problem in general," he adds. —MICHAEL FREEMANTLE HTTP://WWW.CEN-ONLINE.ORG
