Materials imperfections in planar superconducting quantum circuits—in particular, two-level-system (TLS) defects—contribute significantly to decoherence, ultimately limiting the performance of quantum computation and sensing. The identification of specific parasitic layers and their associated loss contributions has, however, proven elusive. Using a combination of x-ray photoemission spectroscopy (XPS) and analytical scanning transmission electron microscopy (STEM), we determine the thickness, chemical composition, and location of the oxides present in niobium-on-silicon coplanar-waveguide (CPW) resonators and quantify their respective contributions to the measured single-photon quality factor ($Q$). Using selective chemical etching, we reduce first the substrate-air oxide then the metal-air oxide thickness, dramatically reducing both TLS (${\delta }_{\mathrm{TLS}}$) and non-TLS (${\delta }_{\mathrm{hi}}$) losses, resulting in a median $Q$ value over $5\times {10}^6$, with individual devices approaching $6\times {10}^6$. We find that silicon surface oxides host 70% of TLS losses, with a ${\delta }_{\mathrm{TLS}}:{\delta }_{\mathrm{hi}}$ loss-density ratio near 11:1. In contrast, niobium surface oxides host 77% of non-TLS losses, uniformly distributed within the oxide layer, with a ${\delta }_{\mathrm{TLS}}:{\delta }_{\mathrm{hi}}$ loss-density ratio of 3:4 for the superconducting circuits investigated in this work. Only 7% of losses come from other sources, including the niobium-silicon interface, which is sufficiently clean in our devices to allow epitaxial $\mathrm{Nb}$ nucleation on $\mathrm{Si}$. As we mitigate surface losses through selective modification of the interface dielectrics, we arrive in a regime where TLS losses are no longer dominant, which will allow other types of losses in superconducting circuits to be investigated in more detail, including nonequilibrium quasiparticles.

In planar superconducting circuits, decoherence due to materials imperfections, especially two-level-system (TLS) defects at different interfaces, is a primary hurdle for advancing quantum computing and sensing applications. Traditional methods for mitigating TLS loss, such as etching oxide layers at metal and substrate interfaces, have proven to be inadequate due to the persistent challenge of oxide regrowth. In this work, we introduce a novel approach that employs molecular self-assembled monolayers (SAMs) to chemically bind at different interfaces of superconducting circuits. This technique is specifically tested here on coplanar waveguide (CPW) resonators, in which this method not only impedes oxide regrowth after surface etching but can also tailors the dielectric properties at different resonators interfaces. The deployment of SAMs results in a consistent improvement in the measured quality factors across multiple resonators, surpassing those with only oxide-etched resonators. The efficiency of our approach is supported by microwave measurements of multiple devices conducted at millikelvin temperatures and correlated with detailed X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) characterizations of SAM-passivated resonators. The compatibility of SAMs materials with the established fabrication techniques offers a promising route to improve the performance of superconducting quantum devices.

In the pursuit of sustainable solutions to the ever-increasing demand for renewable energy, mechanically compliant thermoelectric generators (TEGs) have garnered significant attention owing to the promise they present for application in generating power from waste heat in mechanically challenging scenarios. This review paper examines the ongoing advancements in the efficiency and applicability of TEGs through novel material engineering and design innovations. It delves into the improvement of their thermoelectric (TE) properties via micro- and nanostructural modifications and explores architectural advancements aimed at enhancing functionality and power output. Notably, the integration of TEGs into flexible, stretchable, and wearable electronics has been a significant development, expanding their applications in various domains such as healthcare monitoring, remote sensing, and consumer electronics. The review emphasizes the critical interplay between electronic, thermal, and mechanical aspects in optimizing TEGs performance. By providing an in-depth exploration of these multifaceted interactions and highlighting the significant advancements in materials and design, this review aims to underscore the importance of TEGs in a cleaner and more efficient era of energy generation, with a particular focus on their emerging applications across diverse fields.