A Combined NMR and DFT Study of Narrow Gap Semiconductors: The Case of PbTe

• Published in: Journal of physical chemistry. C Vol. 117; no. 17; pp. 8959 - 8967
• Main Authors: Taylor, Robert E, Alkan, Fahri, Koumoulis, Dimitrios, Lake, Michael P, King, Daniel, Dybowski, Cecil, Bouchard, Louis-S
• Format: Journal Article
• Language: English
• Published: Columbus, OH American Chemical Society 02.05.2013
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Electron states; Exact sciences and technology; Low-dimensional structures (superlattices, quantum well structures, multilayers): structure, and nonelectronic properties; Materials science; Methods of electronic structure calculations; Nanocrystalline materials; Nanoscale materials and structures: fabrication and characterization; Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals; Physics; Structure of solids and liquids; crystallography; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Temperature dependence; Nanostructures; Nanometer scale; Chemical shift; Quantity ratio; Spin-lattice relaxation; p type semiconductor; Resonance frequency; Density functional method; Ab initio calculations; Nuclear magnetic resonance; Carrier density; Nanostructured materials; Nanocrystal; Crystal structure
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp3101877

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 a 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 207Pb and 125Te NMR spin–lattice relaxation rates and NMR shifts for bulk and nanoscale PbTe. The shifts are compared with calculations of the 207Pb and 125Te 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 207Pb and 125Te 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.