Structural study by solid-state 71Ga NMR of thin film transistor precursors

Abstract

Solid-state ⁷¹Ga NMR was used to investigate the structures of several heterometallic Group 13 hydroxo-aquo clusters, [Ga_13−xIn_x (μ₃-OH)₆(μ₂-OH)₁₈(H₂O)₂₄](NO₃)₁₅ which are envisioned for thin film transistors. The characterization of these clusters in the solid state provides additional information in understanding the synthesis, structure and speciation of these precursors for high-quality, ultrasmooth thin films. Yet important structural information regarding these clusters – including the exact composition, isomeric structure, and coordination environments – were unknown prior to this precise NMR spectroscopy study. These molecular species, termed “Ga_13−xIn_x”, contain three types of six-coordinate metal sites, with bridging OH⁻ groups and H₂O as capping ligands, and we report results on Ga₇In₆, Ga₈In₅, Ga₁₀In₃, Ga₁₁In₂, Ga₁₂In₁, and Ga₁₃. Utilizing two magnetic fields (13.9 T and 21.1 T), the solid-state NMR spectra were interpreted in conjunction with computational modeling (using CASTEP) and simulation of spectral lineshapes (using Dmfit). The metal sites are best represented as distorted octahedra, and they exhibit a range of quadrupolar couplings and asymmetry parameters, which can be addressed using longitudinal strain analysis. Until now, there has been speculation about the sites for transmetallation within the synthetic cluster community. Here, we show that Ga NMR is a powerful technique to monitor the transmetallation of In for Ga in the Ga_13−xIn_x clusters, specifically substituting in the “outer ring” sites, and not the “core” or “middle ring”.