Thermophoretic deposition and electrical characterization of flame-synthesized carbon nanoparticle thin films

In the last decades, flame studies focused on particle inception mechanisms and the characterization of particle composition and properties have shown that flames operated in fuel-rich conditions produce a broad variety of CNPs with different morphology, chemical, physical, optical and electronic properties which depend on the flame conditions. Thus, flame combustion can be optimized to form tailored CNPs for specific applications [1, 2]. Besides particle properties, the assembly of nanoparticles into a uniform thin film with precise control over chemical and physical properties poses a significant challenge. Various techniques have been developed and studied over the years to create thin films of nanoparticles that can lead to novel applications, as electro-deposition of semiconductor and metal nanoparticles, the deposition of nanoparticle monolayers via the Langmuir-Blodgett technique, sol-gel chemistrybased deposition. Thermophoretic sampling is a technique often used to collect on a substrate isolated particles from flames for TEM or other single particles analysis techniques. This method relies on the thermophoretic forces driving the aerosol particles in the hot gas flame towards a cold surface inserted in the flame. In this work, carbon nanoparticles have been produced in a premixed ethylene-air flame and their potentiality as a medium for electronic applications has been investigated. To this aim, CNPs thin films have been achieved by the direct deposition of the flame-synthesized CNPs on multilayer substrates consisting on highly-doped (500 ?m thick) Silicon layers, thin (200 nm) SiO2 insulating barriers and gold electrodes (drain/source contacts) with interdigitated layout. The flame conditions examined in this work allow to produce CNPs that can roughly be distinguished into two classes: organic particles with diameter, D=2-10 nm, which have molecularlike spectroscopic properties, and soot particles, which are constituted by amorphous carbon in form of primary particles with D=10-50nm which can grow in chain like structures. CNPs have been characterized by measuring the size distribution by a Scanning Mobility Particle Sizer, and their physico-chemical properties by UV-visible light absorption and Raman spectroscopy. Current-voltage (IV) measurements recorded for all the synthesized films display a linear (ohmic) behavior for values of the applied voltages up to 10V. Moreover, it was found that the electrical conductivity (?) of these layers exhibit a distinctive dependence on the thickness following a rising behavior up to a maximum value of about 10-3 S/cm. Finally, by applying a further external voltage signal to the highly-doped Silicon substrate, working as the gate electrode in a field-effect transistor configuration, we have been able to modify reversibly the current flowing in the CNPs channels, thus revealing the possibility to evaluate also the charge carrier mobility (?) through the MOSFET model. Maximum ? values of about 10-4 cm2/volt*sec were estimated in this work.