Synthetic Metals 138 (2003) 267–269
Transport properties of polyoxometalate containing polymeric materials N. Glezosa,*, D. Velessiotisa, G. Chaidogiannosa,b, P. Argitisa, D. Tsamakisb, X. Ziannia a
Institute of Microelectronics, NCSR ‘‘Demokritos’’, Aghia Paraskevi, Athens 15310, Greece b Department of Electrical & Computer Engineering, NTUA, Zographou 15773, Greece
Abstract In this paper, molecular compounds that come from the class of tungsten or molybdenum polyoxometalates (POM) are examined as components of polymeric materials with potential use in nanolithography and molecular devices. The main reason for selecting molecules of this class is their well defined and stable structure as well as their thermal or photochemical reduction–oxidation and catalytic properties. The material used is in the form of POM molecules embedded into a polymeric lithographic resist matrix. This process enables nanopatterning of the conductive channel thus bypassing manufacturing steps. By diminishing electrode dimensions and molecular distance, transport is dominated by tunneling effects. In a specific case negative resistance is observed. A possible explanation is a multiple tunneling mechanism due to the presence of POM molecules. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Polyoxometalate; Tunneling; Molecular electronic devices
1. Introduction The use of molecular materials as active elements in microelectronic devices is a challenging new possibility [1,2]. The idea is to use isolated molecules as active elements and interconnections of devices that perform the basic electronic functions of rectification, amplification and information storage, thus reducing circuit dimensions to the molecular scale. Until this day only a few successful examples of exploitation of the molecular properties for the fabrication of switching devices operating at room temperature conditions have appeared [3,4], the most recent being a transistor device based on a molecular monolayer. In this paper, we perform a preliminary investigation of the transport properties of tungsten polyoxometalates (POM). These molecules are known for their well defined and stable structure as well as their thermal or photochemical reduction–oxidation, and catalytic properties [5–8]. Due to their structural resemblance to semiconductor materials they can be considered as zero dimension semiconductors. POMs have been also used as photosensitizers in poly(vinyl alcohol) (PVA) based resist formulations for bilayer DUV and X-ray microlithography almost 10 years ago [9,10]. More recently the formation of POM monolayers * Corresponding author. Tel.: þ30-10-650-3236; fax: þ30-10-651-1723. E-mail address:
[email protected] (N. Glezos).
using the Lagmuir–Blodgett technique [11] has also been reported. The aim of this work is to embed molecules of this class in a suitable substrate in order to obtain a composite material for molecular switching device applications which at the same time possesses lithographic capability in the nanometer region. If this succeeds, the patterning of molecular channels will be greatly simplified since no further resist processing will be needed.
2. Results and discussion 2.1. Lithography Different formulations were tested targeting at a combination of successful lithography and a stable final material after processing [12]. The one important requirement is that the guest POM molecule should not interact chemically with the polymer guest material. Furthermore the POM concentration should not be altered after the process. Two types of host materials were evaluated: (a) poly(vinyl alcohol) based formulations, very similar to the ones that had been used in the past [9,10] and (b) poly(methyl methacrylate) (PMMA) based formulations. Both classes were tested for their lithographic resolution capability under e-beam exposure. In the case of PVA/POM resist formulations, line features
