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

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 nand 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. 2

Nanoscale β-nuclear magnetic resonance depth imaging of topological insulators

“…S5). Other detailed NMR studies of the Knight shift and relaxation in PbTe (E g = 0.32 eV, 300 K) and Tl 2 Se (E g = 0.6 eV, 300 K) were consistent with a decrease in carrier concentration in nanoscale materials compared with bulk materials (24,25), in contrast with the behavior observed in TI nanocrystals. Furthermore, in the case of ZnTe (E g = 2.23 eV, 300 K) the 125 Te frequency shift of nanoZnTe remains unshifted relative to micrometer-sized samples (Fig.…”

Site‐Specific Contributions to the Band Inversion in a Topological Crystalline Insulator

“…8d) for SnTe (x=1). This is an interesting result given that the SnTe is also a p-type semiconductor (with a lower Seebeck coefficient than PbTe) and this sort of temperature dependence is usually observed for n-type semiconductors, not p-type [12,13,[20][21][22][23][24][25][26] rates, see below). At this Sn fraction the CB and VB are now overlapped, yielding an increase in the DOS at the Fermi level and a metallic character due to the E g →0 (Fig.…”

“…Such a resonance frequency shift is considered large for a semiconductor. Previous studies have shown that the resonance shifts of 207 Pb can extend up to 12,000 ppm (relative to tetramethyllead [19,42] by extrapolating the resonance position to the limit of zero free carriers [12,21]. If the carrier concentration (Knight shift) were the main contribution to the total shift, then we should expect the 207 Pb spectrum of x=0.60 to be more upfield (lower frequency) than the spectrum of x=0.35.…”

“…They can report on relativistic effects (such as SOC) related to heavy Z-elements in alloys [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. Also, NMR can probe materials that are either p-type or ntype and also materials containing multiple fractions of different carrier types [12,13]. The influence of SOC on the 125 Te and 77 Se nuclear spin-lattice relaxation times (T 1 ) of the topological insulators (TIs) Bi 2 Te 3 and Bi 2 Se 3 , respectively, was discussed in [26].…”

Two-band model interpretation of the p- to n-transition in ternary tetradymite topological insulators

“…activated T-dependence [37]. NMR in PbSe [38] and PbTe [39] has probed similar behavior. At ambient temperature, the interplay between and ℎ is constant as expressed by the conductivity but strongly varies across the Bi 2 Te 3-x Se x series.…”