A combined NMR and DFT study of Narrow Gap Semiconductors: The case of PbTe

TL;DR

This paper combines experimental NMR and ab initio calculations to distinguish chemical and Knight shifts in PbTe semiconductors, providing a more efficient method for analyzing carrier effects and exploring nanocrystal properties.

Contribution

It introduces an alternative approach using NMR and DFT calculations to separate chemical and Knight shifts in PbTe, reducing the need for multiple sample studies.

Findings

Good agreement with literature on chemical shifts at zero carrier concentration

Evidence supporting the 'self-cleaning effect' in PbTe nanocrystals

Comparison of NMR shifts with literature Knight shift data

Abstract

In this study we present an alternative approach to separating contributions to the NMR shift originating from the Knight shift and chemical shielding by a combination of experimental solid-state NMR results and ab initio calculations. The chemical and Knight shifts are normally distinguished through detailed studies of the resonance frequency as function of temperature and carrier concentration, followed by extrapolation of the shift to zero carrier concentration. This approach is time-consuming and requires studies of multiple samples. Here, we analyzed $^{207}$Pb and $^{125}$Te NMR spin-lattice relaxation rates and NMR shifts for bulk and nanoscale PbTe. The shifts are compared with calculations of the $^{207}$Pb and $^{125}$Te chemical shift resonances to determine the chemical shift at zero charge carrier concentration. The results are in good agreement with literature values from carrier concentration-dependent studies. The measurements are also compared to literature reports of the $^{207}$Pb and $^{125}$Te Knight shifts of $n$- and $p$-type PbTe semiconductors. The literature data have been converted to the currently accepted shift scale. We also provide possible evidence for the "self-cleaning effect" property of PbTe nanocrystals whereby defects are removed from the core of the particles, while preserving the crystal structure.