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A new transistor concept exploits the colloidal synthesis of metal nanoparticles and their Coulomb charging energy.

Electrical transport properties were measured on the recently discovered ferromagnetic spinels CdCr2S4 (Tc=85°K) and CdCr2Se4 (Tc=130°K). The large Cr−Cr separation (≥3.63 Å) excludes metallic conductivity due to Cr−Cr overlap. Electrical conductivity, Seebeck effect, Hall effect, and magnetoresistance measurements were made on high-density, polycrystalline samples (CdCr2S4: 99.6%; CdCr2Se4: 99.9% of the theoretical density). Both materials show a negative temperature coefficient of the resistivity in the para- and ferromagnetic region without any discontinuity at the Curie temperature. The room-temperature conductivities of CdCr2S4 and of CdCr2Se4 are 5×10−4 (Ω·cm)−1. At 77°K the conductivity of CdCr2S4 is 3.5×10−10 (Ω·cm)−1 and of CdCr2Se4 6.7×10−5 (Ω·cm). The Seebeck coefficient for both materials is rather small: (CdCr2S4: −60 μV/°K; CdCr2Se4: +60 μV/°K), which indicates that the materials are almost compensated. The room-temperature mobility of CdCr2Se4 is 7 cm2/Vsec. In the ferromagnetic region CdCr2Se4 exhibits a negative magnetoresistance effect. An applied field of 7 kG results in a 4% increase of the conductivity. Band formation in CdCr2Se4 due to overlap of the 4p orbitals of the Se ions is proposed.

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Metallic nanoparticles offer possibilities to build basic electric devices with new functionality and improved performance. Due to the small volume and the resulting low self-capacitance, each single nanoparticle exhibits a high charging energy. Thus, a Coulomb-energy gap emerges during transport experiments that can be shifted by electric fields, allowing for charge transport whenever energy levels of neighboring particles match. Hence, the state of the device changes sequentially between conducting and non-conducting instead of just one transition from conducting to pinch-off as in semiconductors. To exploit this behavior for field-effect transistors, it is necessary to use uniform nanoparticles in ordered arrays separated by well-defined tunnel barriers. In this work, CoPt nanoparticles with a narrow size distribution are synthesized by colloidal chemistry. These particles are deposited via the scalable Langmuir-Blodgett technique as ordered, homogeneous monolayers onto Si/SiO2 substrates with pre-patterned gold electrodes. The resulting nanoparticle arrays are limited to stripes of adjustable lengths and widths. In such a defined channel with a limited number of conduction paths the current can be controlled precisely by a gate voltage. Clearly pronounced Coulomb oscillations are observed up to temperatures of 150 K. Using such systems as field-effect transistors yields unprecedented oscillating current modulations with on/off-ratios of around 70%.