Optically induced insulator-to-semiconductor transition in fluorescent carbon quantum dots measured by terahertz time-domain spectroscopy

Abstract

The fluorescent carbon quantum dots (FCQDs) are promising for wide applications especially in the field of optoelectronic materials and devices. For these applications, it is essential to examine the optoelectronic properties of FCQDs not only in a solution but also in the form of dry particles. Here we study the optoelectronic properties of dry FCQDs by using terahertz (THz) time-domain spectroscopy (TDS). The THz response of dry FCQDs, which can emit blue (B-CQDs) and red (R-CQDs) lights in solutions under optical excitation, has been measured in temperature range from 80 to 280 K. We find that R-CQDs behaves like an optical insulator in 0.2–1.2 THz range, while B-CQDs experiences insulator-to-semiconductor transition with increasing THz frequency and temperature. The optical conductance and the key sample parameters of B-CQDs are determined. This is the first research work in applying the THz TDS to study the dry CQDs. The findings from this study can be helpful in gaining an in-depth understanding of the physical properties of dry FCQDs.