Evidence for size effect on insulator-metal transition in C: (H; S)

Abstract

Temperature dependent electrical resistivity, ϱ( T), measurements have been carried out on nano-crystalline C : (H; S) obtained by the pyrolysis of p-polyphenylene sulfide (PPS) between 600–1100°C. At low heat treatment temperatures (HTT 600–700°C) X-ray diffraction and EPR spectroscopy give evidence for the formation of condensed polycyclic benzenoid units of ∼1 nm dimension having a charge carrier density ∼10 19 cm −3. In this regime, where n c 1 3 a H ∗ < 0.26 ( n c is the critical number of free carriers per unit volume and a H ∗ is the effective Bohr radius) the samples are insulating. When the HTT is increased to 800–1100°C an increase in the effective length scale of the system from 1 nm to 5–15 nm occurs with a concomitant increase in conductivity σ 300 by five orders of magnitude. This suggests that the transition is primarily due to an increase in the effective length scale or a H ∗ , and the change in carrier density as well as disorder play a minor role, which gives compelling evidence for quantum size effect on Mott-Anderson type transition in this system.