EVIDENCE FOR SIZE EFFECT ON INSULATOR-METAL TRANSITION IN C(H,S)

Temperature dependent electrical resistivity, rho(T), measurements hav e been carried out on nano-crystalline C:(H;S) obtained by the pyrolys is of p-polyphenylene sulfide (PPS) between 600-1100 degrees C. At low heat treatment temperatures (HTT 600-700 degrees C) X-ray diffraction and EPR spectroscopy give evidence for the formation of condensed pol ycyclic benzenoid units of similar to 1 nm dimension having a charge c arrier density similar to 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 insulat ing. When the HTT is increased to 800-1100 degrees C an increase in th e effective length scale of the system from 1 nm to 5-15 nm occurs wit h a concomitant increase in conductivity sigma(300) by five orders of magnitude. This suggests that the transition is primarily due to an in crease in the effective length scale or a(H), and the change in carri er density as well as disorder play a minor role, which gives compelli ng evidence for quantum size effect on Mott-Anderson type transition i n this system. Copyright (C) 1996 Elsevier Science Ltd