Carrier mobility of polycrystalline-amorphous carbon nanocoils and its distribution in length direction

Abstract

Polycrystalline-amorphous carbon nanocoils (CNCs) have been discovered and studied for years. Herein, carrier mobility (μ) and concentration (n) of CNCs are probed for the first time, using electric field regulation and finite element analysis, which are calculated to be 21 cm2/Vs and 1.6 × 1019 cm−3 at room temperature. Temperature dependence characterization from 290 to 350 K reveals 0.8–1.6 folds increase of μ, but only 5%∼14% increase of electrical conductivity. This determines a significant decrease of n with temperature increasing. Variable X-ray diffraction characterization attributes this illogical decrease of n to the thermal expansion in c-axis direction of sp2 grains, when there are abundant vacancies in CNCs providing spaces for the expansion. Upon expansion, the compression of the space between sp2 grains results in reduced carrier scattering and increased μ. The weakened atomic interaction in a sp2 grain increases band gap and decreases density of state, determining the decrease of n. Four-electrode characterization along the length direction of single CNCs uncovers a decrease (increase) tendency of μ (n) along with the growth process, which clarifies a negative (positive) relation between defect level and μ (n), and predicts the potential optoelectronic applications of CNCs.