Size-dependent properties ofTl2Sestudied by NMR spectroscopy

Abstract

We report on size-dependent properties of dithallium selenide, ${\mathrm{Tl}}_{2}\mathrm{Se}$. We have carried out a comparative nuclear magnetic resonance (NMR) study of ${\mathrm{Tl}}_{2}\mathrm{Se}$ nanorods and bulk samples, measuring NMR spectra and spin-lattice relaxation rate of $^{203}\mathrm{Tl}$ and $^{205}\mathrm{Tl}$ isotopes. Though bulk ${\mathrm{Tl}}_{2}\mathrm{Se}$ was reported to be a metal, the Korringa-like spin-lattice relaxation behavior is observed only at low temperatures and is transformed to an activation regime above $\ensuremath{\sim}200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This finding is interpreted assuming a two-band model in the semimetallic compound. Our measurements show significant difference in the Knight shift and indirect nuclear exchange coupling for the bulk and nanorod ${\mathrm{Tl}}_{2}\mathrm{Se}$ samples, reflecting noticeable distinction in their electronic structure. At that, ${\mathrm{Tl}}_{2}\mathrm{Se}$ nanorods are semiconductors and exhibit a characteristic activation behavior in the spin-lattice relaxation rate due to the thermal excitation of carriers to the conduction band. The obtained size dependence of the ${\mathrm{Tl}}_{2}\mathrm{Se}$ properties is interpreted in terms of the semimetal-semiconductor transformation due to the quantum confinement.