Electron transport in semiconducting nanoparticle and nanocluster carbon–polymer composites Pandiyan Murugaraj a , David Edward Mainwaring a, * , Timur Jakubov a , Nelson Eduardo Mora-Huertas a , Nurra Ali Khelil a , Rainer Siegele b a School of Applied Sciences, Royal Melbourne Institute of Technology, GPO Box 2476V, Melbourne, Vic. 3001, Australia b Australian Nuclear Science Technology Organisation, Private Mail Bag 1, Menai, NSW 2234, Australia Received 16 December 2005; accepted 18 December 2005 by A.H. MacDonald Available online 10 January 2006 Abstract The electron transport properties of two types of carbon–polyimide (C–PI) nanocomposite thin films have been evaluated. Conductive nanocomposites formed by incorporation of 30 nm carbon particles prior to polymer cross linking (ex situ formation) has been compared to high energy ion beam irradiation in situ formation of nanoscale carbon clusters within the polymer composite. Addition of carbon nanoparticles were able to reduce the resistivity by 13 orders of magnitude for 8 vol% carbon content. The irradiated in situ formed film showed a comparable resistivity to this 8% C–PI film. All the films exhibited negative temperature coefficient of resistance (NTCR) behaviour. While in the ex situ films the NTCR decreased progressively with increasing temperature above 350 K, the in situ film exhibited a constant NTCR value at ambient as well as elevated temperatures indicating that films formed by ion beam irradiation eliminate possible clustering of nanoparticles prior to crosslinking seen in the ex situ films. The optimum hop energies for the ex situ films ranged from 23.1 to 8.05 meV when carbon content increased from 1 to 8 vol% and the corresponding value for the in situ formed film was 34.94 meV. These films had appreciable NTCR values, and were evaluated for their thermistor behaviour as a class of material with potential for temperature sensing devices. q 2006 Elsevier Ltd. All rights reserved. PACS: 72.80Tm; 81.05Qk; 84.32Ff Keywords: A. Conducting polymer nanocomposites; A. Thermistors; D. Electron transport 1. Introduction The use of polymer based electronics has generated a significant interest over the last few years, since it holds the promise of flexible light weight devices, improved device packaging, as well as lower cost and greater flexibility in fabrication compared to conventional silicon based technology [1]. Recent research attention has been focussed on the organic molecular systems comprising small molecule and oligomer assemblies (molecular electronics) and conjugated polymer networks (intrinsically conductive polymers). Molecular-scale electronic responses may also be gained through interactions at the interface between nanoscale particles and the chemical functionality of dielectric polymers. Conductive nanocomposite polymers consisting of a non- conducting medium and conductive nanoparticles such as carbon are widely used in electromagnetic interference shielding, electrostatic dissipation, embedded passive com- ponents and positive temperature coefficient elements such as switching devices [2–5]. Many applications of polymer based electronics involve extreme conditions such as defence and industry electronics that require high endurance to elevated temperatures and mechanical resilience. To date, certain polyimides provide superior thermal stability while maintain- ing excellent mechanical properties under such conditions [6]. Conductive nanocomposites may be formed by incorporation of nanoparticles within the polymer medium by dispersion into the monomers or oligomers prior to the cross-linking reactions (ex situ formation) or formed by ion beam irradiation of the preformed polymer films (in situ formation). In this paper, we provide studies on the electron transport properties of both types of nanocomposite thin films operating in a compostional range that provides semiconducting behaviour. Solid State Communications 137 (2006) 422–426 www.elsevier.com/locate/ssc 0038-1098/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.12.027 * Corresponding author. Tel.: C61 3 9925 4784; fax: C61 3 9639 1321. E-mail address: david.mainwaring@rmit.edu.au (D.E. Mainwaring).