Electrical Percolation Threshold and Size Effects in Polyvinylpyrrolidone-Oxidized Single-Wall Carbon Nanohorn Nanocomposite: The Impact for Relative Humidity Resistive Sensors Design.

Bogdan-Catalin Serban,Matei Serbanescu,Cristina Pachiu,Cornel Cobianu,Cosmin Cobianu,Mihai Brezeanu,Cristina Mihaela Nicolescu,Niculae Dumbravescu,Marius Bumbac,Octavian Buiu

doi:10.3390/s21041435

Abstract

This paper reports, for the first time, on the electrical percolation threshold in oxidized carbon nanohorns (CNHox)–polyvinylpyrrolidone (PVP) films. We demonstrate—starting from the design and synthesis of the layers—how these films can be used as sensing layers for resistive relative humidity sensors. The morphology and the composition of the sensing layers are investigated through Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and RAMAN spectroscopy. For establishing the electrical percolation thresholds of CNHox in PVP, these nanocomposite thin films were deposited on interdigitated transducer (IDT) dual-comb structures. The IDTs were processed both on a rigid Si/SiO2 substrate with a spacing of 10 µm between metal digits, and a flexible substrate (polyimide) with a spacing of 100 µm. The percolation thresholds of CNHox in the PVP matrix were equal to (0.05–0.1) wt% and 3.5 wt% when performed on 10 µm-IDT and 100 µm-IDT, respectively. The latter value agreed well with the percolation threshold value of about 4 wt% predicted by the aspect ratio of CNHox. In contrast, the former value was more than an order of magnitude lower than expected. We explained the percolation threshold value of (0.05–0.1) wt% by the increased probability of forming continuous conductive paths at much lower CNHox concentrations when the gap between electrodes is below a specific limit. The change in the nanocomposite’s longitudinal Young modulus, as a function of the concentration of oxidized carbon nanohorns in the polymer matrix, is also evaluated. Based on these results, we identified a new parameter (i.e., the inter-electrode spacing) affecting the electrical percolation threshold in micro-nano electronic devices. The electrical percolation threshold’s critical role in the resistive relative-humidity sensors’ design and functioning is clearly emphasized.

Highlights

The transport properties of composite materials, those related to electrical transport, have been intensively investigated over the past 20 years
This paper reports on the design and synthesis of oxidized single-wall carbon nanohorns
The sensing layers’ morphology and qualitative composition were investigated through Atomic Force Microscopy (AFM), Raman spectroscopy, and Scanning Electron

Summary

Introduction

The transport properties of composite materials, those related to electrical transport, have been intensively investigated over the past 20 years. The lowest filler concentration value at which the insulating material is converted into a conductive composite is called percolation threshold, ψc [4,5] Thanks to their high electrical conductivity and outstanding mechanical properties, the electrical percolation threshold of polymeric composites incorporating carbon nanotubes (CNTs) have received particular attention. This important characteristic of the nanocomposite depends on many parameters; among them, one can mention: the CNT type (SWCNT–single-walled carbon nanotube, MWCNT–multi-walled carbon nanotube), the synthesis method (either chemical vapor deposition, arc discharge, or laser ablation), the type of pre-chemical treatment (thermal oxidation, UV/O3, HNO3), functionalization (NH2, polyphenylene ethynylene), size and geometry of aggregates, polymer class, etc. Slow or medium stirring yields kinetic percolation (the particles are free to move and thereby can form a conducting network at much lower particle concentrations) [11]

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