Semiconducting quasi 1-dimensional Metal Compounds

Realization of the often-made promise of mass-produced, low-cost "plastic electronics" based on semi-conducting organic materials [1] requires, among other qualities, ease of processing and stability during fabrication and practical use of the final products. Virtually all organic polymers and oligomers that are under current scrutiny for this purpose appear to suffer from degradation upon exposure to oxygen and water, and, hence, require manufacturing conditions, as well as packaging systems such as glass [2] that are prone to eliminate at least some of their purported benefits. In a broad international cooperation, we produced and characterized (oriented) thin films, fibers and field-effect transistors (FETs), comprising semi-conducting, platinum-based chain-structures synthesized in aqueous media, that were processed under ambient conditions from common organic solvents, and exposed -without significant loss of performance- to white light and air for periods of time in excess of 6 months. Remarkably, immersion of the FETs in water of 90°C for more than 12 hrs did not deteriorate important device characteristics, but, in fact, improved for instance their ON-OFF switching ratios by a factor 10 and more.

Figure 1. a. Chemical structure of tetrakis((S)-1-amino-3,7-dimethyloctane) platimum(II)-tetrachloroplatinate(II), [Pt(NH₂dmoc)₄][PtCl₄]. b. Wide-angle X-ray diffraction patterns showing that the crystalline order in the Pt-compound was stable up to ~140°C.

Our novel semi-conducting material is a quasi-one dimensional chain-structure with a backbone of linearly arranged platinum atoms (Fig. 1). It is based on Magnus’ green salt, [Pt(NH₃)₄][PtCl₄], that was described as long ago as 1828 [3]. Unfortunately, the early Pt-compounds are intractable and, as a result, have found no practical applications. However, by carefully selecting the chemical structure of the ligands, e.g. NH₂R with R a linear or branched alkyl group, it has proven possible to synthesize soluble [Pt(NH₂R)₄][PtCl₄] compounds [4,5]. Particularly beneficial properties are found for the derivative in which R is (S)-3,7-dimethyloctyl (dmoc). The latter material is highly soluble at 70-80°C in common organic solvents such as toluene, from which the Pt-compound can conveniently be recrystallized by cooling or evaporation of the solvent. This very desirable property makes it possible to readily form films, fibers (Fig. 2), blends with polymers, and other structures.

Figure 2. a. Polarized optical micrograph of an oriented filament of [Pt(NH₂dmoc)₄][PtCl₄] produced by electro-spinning. b. Electron diffraction pattern of a film revealing the extraordinary degree of orientation of the Pt-compound. c,d. Scanning probe microscopy images suggestive of the helical nature of [Pt(NH₂dmoc)₄][PtCl₄]. c. Original image taken in deflection mode; inset: fast-Fourier transform. d. FFT-filtered image of c; inset: FFT-filtered height image.

Simple, field-effect transistors comprising [Pt(NH₂dmoc)₄][PtCl₄] as the active semi-conductor layer were produced under ambient conditions in air with highly oriented films grown onto PTFE orientation layers (Fig. 3). Devices in which the Pt-chain structures were aligned parallel to the current transport direction exhibited p-type transistor action with field-effect mobilities on the order of 10^-3-10^-4 cm²/Vs. No detectable differences were observed between as-prepared devices and those stored at ambient atmosphere and light for periods up to 6 months.

Figure 3. a,b. Optical micrograph (a) and schematic (b) of a thin-film field-effect transistor (FET) comprising [Pt(NH₂dmoc)₄][PtCl₄] (aligned on highly-oriented poly(tetrafluoroethylene) PTFE 3) as the active semi-conducting layer 4, vacuum-evaporated gold source/drain electrodes 5, and an n^{^2+}-doped silicon wafer 1 with a 200-nm thin oxide layer 2 as gate and gate insulator, respectively. c. Transfer characteristic of as-produced device. d. Characteristic of the same device, but stored for 12 hrs in water at 80°C. (Insets: Corresponding logarithmic plots.) e. Output characteristics of [hot-]water-treated FETs, comprising highly oriented (graph) and spin-coated, unoriented (inset) [Pt(NH₂dmoc)₄][PtCl₄] active layers. f. Relationship of field-effect mobility, m_FET, and conductivity, s, as determined from FET device characteristics. Squares and triangles represent data taken for FETs based on aligned [Pt(NH₂dmoc)₄][PtCl₄], channel parallel to Pt-chains: open squares, as-prepared devices; solid black symbols, devices of different batches after various temperature and kinetic studies, but before H₂O-bath; solid blue symbols, devices after hot-water treatment. Open black circles are data points for devices with channel perpendicular to chains, before water storage; open blue circle: spin-coated device, after water treatment. For comparison, the power-law relationship collected by Brown et al.[6] for various amorphous organic semi-conductor FETs is also included (red curve).

In as-prepared devices a film (bulk) conductivity on the order of 10^-7 S/cm was observed (Fig. 3c,f). Unpackaged transistors were immersed in water at a temperature of up to 90°C for a period of 12 hrs and more (Fig. 4), followed by drying. Afterwards a decrease of the film conductivity by about one order of magnitude was observed resulting in an increase of the ON-OFF current ration to >10²-10³ (inset Fig. 3d). Most remarkably, the transistor devices showed no evidence of degradation demonstrating the extraordinary stability of [Pt(NH₂dmoc)₄][PtCl₄] (Fig. 3d,e).

Figure 4. Array of 8 field-effect transistors being subjected to hot-water treatment.

The very simple and versatile synthesis, convenient processibility, and outstanding resistance to relatively harsh environmental conditions, could make compounds of the type of the present [Pt(NH₂dmoc)₄][PtCl₄] the material of choice for certain “sloppy” electronic products.

Authors

W. R. Caseri^a, H. D. Chanzy^ab, K. Feldman^a, M. Fontanaa, Paul Smith^a, T.A. Tervoort^a, J. G. P. Goossens^c, E. W. Meijer^c, A. P. H. J. Schenning^c, I. P. Dolbnya^d, M. G. Debije^c, M. P. de Haas^c, J. M. Warman^c, A. M. van de Craats^e, R. H. Friend^e, H. Sirringhaus^e & N. Stutzmann^e

^aDepartment of Materials, ETH Zürich
^bC.E.R.M.A.V., CNRS, Grenoble
^cEindhoven University of Technology
^dDUBBLE/ESRF, Grenoble
^eDelft University of Technology
^fUniversity of Cambridge, Cavendish Laboratory

References

1. A.J. Heeger, Rev. Mod. Phys. 2001, 73(3), 681.
2. K. Pichler, Phil. Trans. R. Soc. London A 1997, 355, 829.
3. G. Magnus, Pogg. Ann. 1828, 14, 239.
4. J. Bremi, W. Caseri, P. Smith, J. Mater. Chem. 2001, 11, 2593.
5. M. Fontana et al., Chem. Mat. 2002, 14, 1730.
6. A.R. Brown, D. M. de Leeuw, E. E. Havinga, A. Pomp, Synth. Met. 1994, 68, 65.