Spatially correlated distributions of local metallic properties in bulk and nanocrystalline GaN

Abstract

We compare local electronic structure at different atom types of a metallic semiconductor in bulk and nanocrystalline form. Multinuclear magic-angle-spinning nuclear magnetic resonance (MAS NMR) establishes that GaN synthesized as an intentionally-doped bulk powder or as annealed nanocrystalline particles exhibits metallic behavior and a wide distribution of differing electronic environments in both forms. Bulk polycrystalline wurtzite GaN doped with 0.13% Ge as a shallow donor exhibits a temperature-independent distribution of 71 Ga Knight shifts over the temperature range 123-473 K. Each Knight shift frequency in the inhomogeneously-broadened spectrum is characterized by a 71 Ga spin-lattice relaxation time T 1 that is in good agreement with the value predicted by the Knight-Korringa relation across the broad range of temperatures. The14 N spectrum shows a slightly smaller Knight shift distribution with spin-lattice relaxation time T 1 values at 295 K across the distribution also in good agreement with the Knight-Korringa relation. Similarly, annealed nanocrystalline wurtzite GaN (50-100 nm, and without Ge) exhibits a 71 Ga Knight shift distribution and T 1values (at 295 K) that follow the same Knight-Korringa behavior. Thus, both bulk and nanocrystalline forms of GaN are n-type and well above the metal-insulator transition (MIT), the nanocrystals most likely as a result of incorporation of shallow donor oxygen atoms during synthesis. Carriers in both forms of sample exhibit the near-ideal characteristics of a degenerate Fermi gas of non-interacting spins. The observation of NMR signals from both atom types, Ga and N, allows for the direct spatial correlation of the local electronic structure at the two sites in the lattice, specifically the s-orbital character of the electronic wavefunction of conduction band electrons at the Fermi edge. The relative values of these carrier wavefunction probabilities (nearly twice as great for the N atom as for the Ga) are in line with theoretical predictions. Analyses of 71 Ga, 14 N, and 15 N NMR results, including double-resonance 2D 15 N{ 71 Ga} measurements, reveal electronic disorder in the form of broad distributions of local metallic properties (Knight shifts) that are shown to be spatially correlated on a sub-nanometer scale.

Electrochemical Oxidative Fluorination of an Oxide Perovskite

“…40,41 Interestingly, all of the other signals correspond to much shorter 19 F T 1 values of 0.3-0.4 s (Table T3), consistent with fluoride environments that are influenced by conducting or donor electrons. [40][41][42] DFT calculations of several structural models selected from the phase diagram for fluoride intercalation into ReO 3 predict an isotropic 19 F chemical shift of -141 ppm for mono-ReO 3 F, close to the values of several of the measured signals. By comparison, the DFT calculations predict the isotropic chemical shift of a model structure with 7-coordinate distorted-octahedra to be near -73 ppm and of tet-FReO 3 to be near -11 ppm, neither of which are experimentally observed (Figure S10).…”

Electrochemical Oxidative Fluorination of an Oxide Perovskite

“…40,41 Interestingly, all of the other signals correspond to much shorter 19 F T 1 values of 0.3-0.4 s (Table T3), consistent with fluoride environments that are influenced by conducting or donor electrons. [40][41][42] DFT calculations of several structural models selected from the phase diagram for fluoride intercalation into ReO 3 predict an isotropic 19 F chemical shift of -141 ppm for mono-ReO 3 F, close to the values of several of the measured signals. By comparison, the DFT calculations predict the isotropic chemical shift of a model structure with 7-coordinate distorted-octahedra to be near -73 ppm and of tet-FReO 3 to be near -11 ppm, neither of which are experimentally observed ( Figure S10).…”

Syntheses of Colloidal F:In2O3 Cubes: Fluorine-Induced Faceting and Infrared Plasmonic Response

“…In metallic materials, including degenerately doped semiconductors, 77 nuclear spins and conduction band electrons in s-like orbitals couple through Fermi contact interactions. These interactions give rise to two characteristic effects that are manifested in the 19 F NMR spectra of F:In2O3 NCs: a frequency displacement of the 19 F NMR signals called the Knight shift, [77][78][79] and a Korringa-type temperature-dependence of the rate of 19 F nuclear spinlattice relaxation. 78,80,81 For the ideal case of isolated nuclear spins coupled to a degenerate gas of electron spins, the Korringa contribution to the relaxation rate, T1,K -1 , is related to the Knight shift, K, by the well-known Knight-Korringa relation: 78,80 1,…”