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First-principles calculations of defects and electron–phonon interactions play a critical role in the design and optimization of materials for electronic and optoelectronic devices. The late Audrius Alkauskas made seminal contributions to developing rigorous first-principles methodologies for the computation of defects and electron–phonon interactions, especially in the context of understanding the fundamental mechanisms of carrier recombination in semiconductors. Alkauskas was also a pioneer in the field of quantum defects, helping to build a first-principles understanding of the prototype nitrogen-vacancy center in diamond, as well as identifying novel defects. Here, we describe the important contributions made by Alkauskas and his collaborators and outline fruitful research directions that Alkauskas would have been keen to pursue. Audrius Alkauskas’ scientific achievements and insights highlighted in this article will inspire and guide future developments and advances in the field. 

Gallium oxide when doped with Mg becomes semi-insulating and can be useful for power electronic devices. The present work investigates optical transitions of neutral Mg (MgGa0) using photoinduced electron paramagnetic resonance spectroscopy, a variation of the traditional optical absorption. Steady-state and time-dependent measurements are carried out at 130 K by illuminating the samples with photon energies from 0.7 to 4.4 eV. Interpretation of the data using a model that incorporates electron–phonon coupling yields a defect transition level that is consistent with the MgGa−/0 level obtained from hybrid density functional theory calculations. We conclude that the neutral to negative transition of MgGa that we observe involves an electron transition from the valence band to the defect, and the MgGa−/0 level is located 1.2 eV above the valence band maximum, with a relaxation energy of 1.3 eV. 

We conducted a tip-enhanced Raman scattering spectroscopy (TERS) and photoluminescence (PL) study of quasi-1D TaSe 3− δ nanoribbons exfoliated onto gold substrates. At a selenium deficiency of δ ∼ 0.25 (Se/Ta = 2.75), the nanoribbons exhibit a strong, broad PL peak centered around ∼920 nm (1.35 eV), suggesting their semiconducting behavior. Such nanoribbons revealed a strong TERS response under 785 nm (1.58 eV) laser excitation, allowing for their nanoscale spectroscopic imaging. Nanoribbons with a smaller selenium deficiency (Se/Ta = 2.85, δ ∼ 0.15) did not show any PL or TERS response. The confocal Raman spectra of these samples agree with the previously-reported spectra of metallic TaSe 3 . The differences in the optical response of the nanoribbons examined in this study suggest that even small variations in Se content can induce changes in electronic band structure, causing samples to exhibit either metallic or semiconducting character. The temperature-dependent electrical measurements of devices fabricated with both types of materials corroborate these observations. The density-functional-theory calculations revealed that substitution of an oxygen atom in a Se vacancy can result in band gap opening and thus enable the transition from a metal to a semiconductor. However, the predicted band gap is substantially smaller than that derived from the PL data. These results indicate that the properties of van der Waals materials can vary significantly depending on stoichiometry, defect types and concentration, and possibly environmental and substrate effects. In view of this finding, local probing of nanoribbon properties with TERS becomes essential to understanding such low-dimensional systems.