Superconductor-superconductor bilayers for enhancing single-photon detection.

Here, we optimized ultrathin films of granular NbN on SiO2 and of amorphous αW5Si3. We showed that hybrid superconducting nanowire single-photon detectors (SNSPDs) made of 2 nm thick αW5Si3 films over 2 nm thick NbN films exhibit advantageous coexistence of timing (<5 ns reset time and 52 ps timing jitter) and efficiency (>96% quantum efficiency) performance. We discuss the governing mechanism of this hybridization via the proximity effect. Our results demonstrate saturated SNSPDs performance at 1550 nm optical wavelength and suggest that such hybridization can significantly expand the range of available superconducting properties, impacting other nano-superconducting technologies. Lastly, this hybridization may be used to tune properties, such as the amorphous character of superconducting films.

The Development and Atomic Structure of Zinc Oxide Crystals Grown within Polymers from Vapor Phase Precursors.

Sequential infiltration synthesis (SIS), also known as vapor phase infiltration (VPI), is a quickly expanding technique that allows growth of inorganic materials within polymers from vapor phase precursors. With an increasing materials library, which encompasses numerous organometallic precursors and polymer chemistries, and an expanding application space, the importance of understanding the mechanisms that govern SIS growth is ever increasing. In this work, we studied the growth of polycrystalline ZnO clusters and particles in three representative polymers: poly(methyl methacrylate), SU-8, and polymethacrolein using vapor phase diethyl zinc and water. Utilizing two atomic resolution methods, high-resolution scanning transmission electron microscopy and synchrotron X-ray absorption spectroscopy, we probed the evolution of ZnO nanocrystals size and crystallinity level inside the polymers with advancing cycles─from early nucleation and growth after a single cycle, through the formation of nanometric particles within the films, and to the coalescence of the particles upon polymer removal and thermal treatment. Through in situ Fourier transform infrared spectroscopy and microgravimetry, we highlight the important role of water molecules throughout the process and the polymers' hygroscopic level that leads to the observed differences in growth patterns between the polymers, in terms of particle size, dispersity, and the evolution of crystalline order. These insights expand our understanding of crystalline materials growth within polymers and enable rational design of hybrid materials and polymer-templated inorganic nanostructures.

Mechanical Behavior of Hybrid Thin Films Fabricated by Sequential Infiltration Synthesis in Water-Rich Environment.

Sequential infiltration synthesis (SIS) is an emerging technique for fabricating hybrid organic-inorganic materials with nanoscale precision and controlled properties. Central to SIS implementation in applications such as membranes, sensors, and functional coatings is the mechanical properties of hybrid materials in water-rich environments. This work studies the nanocomposite morphology and its effect on the mechanical behavior of SIS-based hybrid thin films of AlOx-PMMA under aqueous environments. Water-supported tensile measurements reveal an unfamiliar behavior dependent on the AlOx content, where the modulus decreases after a single SIS cycle and increases with additional cycles. In contrast, the yield stress constantly decreases as the AlOx content increases. A comparison between water uptake measurements indicates that AlOx induces water uptake from the aqueous environment, implying a "nanoeffect" stemming from AlOx-water interactions. We discuss the two mechanisms that govern the modulus of the hybrid films: softening due to increased water absorption and stiffening as the AlOx volume fraction increases. The decrease in the yield stress with SIS cycles is associated with the limited mobility and extensibility of polymer chains caused by the growth of AlOx clusters. Our study highlights the significance of developing hybrid materials to withstand aqueous or humid conditions which are crucial to their performance and durability.

Engineering Individual Oxygen Vacancies: Domain-Wall Conductivity and Controllable Topological Solitons.

Nanoscale devices that utilize oxygen vacancies in two-dimensional metal-oxide structures garner much attention due to conductive, magnetic, and even superconductive functionalities they exhibit. Ferroelectric domain walls have been a prominent recent example because they serve as a hub for topological defects and hence are attractive for next-generation data technologies. However, owing to the light weight of oxygen atoms and localized effects of their vacancies, the atomic-scale electrical and mechanical influence of individual oxygen vacancies has remained elusive. Here, stable individual oxygen vacancies were engineered in situ at domain walls of seminal titanate perovskite ferroics. The atomic-scale electric-field, charge, dipole-moment, and strain distribution around these vacancies were characterized by combining advanced transmission electron microscopy and first-principle methodologies. The engineered vacancies were used to form quasi-linear quadrupole topological defects. Significant intraband states were found in the unit cell of the engineered vacancies, proposing a meaningful domain-wall conductivity for miniaturized data-storage applications. Reduction of the Ti ion as well as enhanced charging and electric-field concentration were demonstrated near the vacancy. A 3-5% tensile strain was observed at the immediate surrounding unit cells of the vacancies. Engineering individual oxygen vacancies and topological solitons thus offers a platform for predetermining both atomic-scale and global functional properties of device miniaturization in metal oxides.

Formation and manipulation of domain walls with 2 nm domain periodicity in BaTiO3 without contact electrodes.

Interfaces at the two-dimensional limit in oxide materials exhibit a wide span of functionality that differs significantly from the bulk behavior. Among such interfaces, domain walls in ferroelectrics draw special attention because they can be moved deterministically with external voltage, while they remain at place after voltage removal, paving the way to novel neuromorphic and low-power data-processing technologies. Ferroic domains arise to release strain, which depends on material thickness, following Kittel's scaling law. Hence, a major hurdle is to reduce the device footprint for a given thickness, i.e., to form and move high-density domain walls. Here, we used transmission electron microscopy to produce domain walls with periodicity as high as 2 nm without the use of contact electrodes, while observing their formation and dynamics in situ in BaTiO3. Large-area coverage of the engineered domain walls was demonstrated. The domain-wall density was found to increase with increasing effective stress, until arriving at a saturation value that reflects 150-fold effective stress enhancement. Exceeding this value resulted in strain release by domain rotation. In addition to revealing this multiscale strain-releasing mechanism, we offer a device design that allows controllable switching of domain-walls with 2 nm periodicity, reflecting a potential 144 Tb per inch2 neuromorphic network.

Giant Superelastic Piezoelectricity in Flexible Ferroelectric BaTiO3 Membranes.

Mechanical displacement in commonly used piezoelectric materials is typically restricted to linear or biaxial in nature and to a few percent of the material dimensions. Here, we show that free-standing BaTiO3 membranes exhibit nonconventional electromechanical coupling. Under an external electric field, these superelastic membranes undergo controllable and reversible "sushi-rolling-like" 180° folding-unfolding cycles. This crease-free folding is mediated by charged ferroelectric domains, leading to giant >3.8 and 4.6 µm displacements for a 30 nm thick membrane at room temperature and 60 °C, respectively. Further increasing the electric field above the coercive value changes the fold curvature, hence augmenting the effective piezoresponse. Finally, it is found that the membranes fold with increasing temperature followed by complete immobility of the membrane above the Curie temperature, allowing us to model the ferroelectric domain origin of the effect.