Correlating atomic compositions and optoelectronic properties of nanostructured semiconductors

Zach Berkson
Zach Berkson
Engineering II - 1519

Semiconductors heavily doped with shallow donors become metallic, acquiring useful optoelectronic properties for technological applications including solid-state lighting, photovoltaics, and power electronics. Understanding the spatially-varying electronic structures of heavily-doped semiconductors is challenging because of the electronic and structural disorder induced by incorporation of non-stoichiometric dopant species into the semi-crystalline semiconductor lattice. These challenges are exacerbated for semiconductors with nanoscale dimensions (<100 nm), where surface interactions are of increased importance. Advanced solid-state NMR techniques, correlated with scattering and spectroscopic measurements, enable measurement and identification of the local environments of different species within and at the surfaces of nanostructured semiconductors, providing detailed insights into the atomic-level origins of their macroscopic optoelectronic properties.

Two examples will be presented. In nanocrystalline GaN prepared by ammonolysis of gallium oxide precursors, X-ray diffraction and electron microscopy analyses indicate long-range crystal-like ordering, yet broad solid-state 71Ga and 15N NMR signals indicate high degrees of local atomic disorder. Analyses of the 71Ga and 15N NMR spectra and spin-lattice relaxation times show that the broad 71Ga and 15N NMR signals arise from interactions between conduction-band electrons and 71Ga and 15N nuclei. Furthermore, powerful 2D 15N{71Ga} NMR correlation spectra establish that the local electronic environments of the different atoms in the GaN lattice are spatially correlated on a sub-nanometer scale. The methods and analyses are extended to colloidal F-doped In2O3 nanocrystals, where solid-state 19F and 115In NMR spectra and spin-lattice relaxation times, paired with DFT calculations, establish that F acts both as a shallow donor species within the In2O3 lattice and as a surface additive to direct the particle morphology during synthesis. For both the GaN and F-doped In2O3 materials, the presence of unpaired electrons, manifesting in the solid-state NMR spectra, are correlated to the optical properties of the materials.

If time allows, a conceptual extension will be made to mesoporous N,Fe-containing graphitic carbon materials, which are promising alternatives to Pt/C for applications as fuel cell electrocatalysts. The high electrocatalytic activities of these materials arise from their moderately high conductivities, high heteroatom (N, O, and/or Fe) quantities, and high accessible surface areas. Advanced solid-state 15N NMR techniques, including 2D 13C{15N} NMR correlation spectra, detect and resolve different types of 15N heteroatoms in diamagnetic, paramagnetic, and metallic environments, which are further correlated to the macroscopic electrocatalytic properties.