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Carbon dioxide electroreduction facilitates the sustainable synthesis of fuels and chemicals1. Although Cu enables CO2-to-multicarbon product (C2+) conversion, the nature of the active sites under operating conditions remains elusive2. Importantly, identifying active sites of high-performance Cu nanocatalysts necessitates nanoscale, time-resolved operando techniques3-5. Here, we present a comprehensive investigation of the structural dynamics during the life cycle of Cu nanocatalysts. A 7 nm Cu nanoparticle ensemble evolves into metallic Cu nanograins during electrolysis before complete oxidation to single-crystal Cu2O nanocubes following post-electrolysis air exposure. Operando analytical and four-dimensional electrochemical liquid-cell scanning transmission electron microscopy shows the presence of metallic Cu nanograins under CO2 reduction conditions. Correlated high-energy-resolution time-resolved X-ray spectroscopy suggests that metallic Cu, rich in nanograin boundaries, supports undercoordinated active sites for C-C coupling. Quantitative structure-activity correlation shows that a higher fraction of metallic Cu nanograins leads to higher C2+ selectivity. A 7 nm Cu nanoparticle ensemble, with a unity fraction of active Cu nanograins, exhibits sixfold higher C2+ selectivity than the 18 nm counterpart with one-third of active Cu nanograins. The correlation of multimodal operando techniques serves as a powerful platform to advance our fundamental understanding of the complex structural evolution of nanocatalysts under electrochemical conditions.

Epitaxy is a process by which a thin layer of one crystal is deposited in an ordered fashion onto a substrate crystal. The direct epitaxial growth of semiconductor heterostructures on top of crystalline superconductors has proved challenging. Here, however, we report the successful use of molecular beam epitaxy to grow and integrate niobium nitride (NbN)-based superconductors with the wide-bandgap family of semiconductorssilicon carbide, gallium nitride (GaN) and aluminium gallium nitride (AlGaN). We apply molecular beam epitaxy to grow an AlGaN/GaN quantum-well heterostructure directly on top of an ultrathin crystalline NbN superconductor. The resulting high-mobility, two-dimensional electron gas in the semiconductor exhibits quantum oscillations, and thus enables a semiconductor transistoran electronic gain elementto be grown and fabricated directly on a crystalline superconductor. Using the epitaxial superconductor as the source load of the transistor, we observe in the transistor output characteristics a negative differential resistancea feature often used in amplifiers and oscillators. Our demonstration of the direct epitaxial growth of high-quality semiconductor heterostructures and devices on crystalline nitride superconductors opens up the possibility of combining the macroscopic quantum effects of superconductors with the electronic, photonic and piezoelectric properties of the group III/nitride semiconductor family., Author(s): Rusen Yan (corresponding author) [1]; Guru Khalsa [2]; Suresh Vishwanath [1]; Yimo Han [3]; John Wright [2]; Sergei Rouvimov [4]; D. Scott Katzer [5]; Neeraj Nepal [5]; Brian P. [...]

Author(s): Julia A. Mundy [1]; Charles M. Brooks [2]; Megan E. Holtz [1]; Jarrett A. Moyer [3]; Hena Das [1]; Alejandro F. Rbola [1]; John T. Heron [2, 4]; James [...]

For centuries, practitioners of origami ('ori', fold; 'kami', paper) and kirigami ('kiru', cut) have fashioned sheets of paper into beautiful and complex three-dimensional structures. Both techniques are scalable, and scientists and engineers are adapting them to different two-dimensional starting materials to create structures from the macro- to the microscale (1, 2). Here we show that graphene (3-6) is well suited for kirigami, allowing us to build robust microscale structures with tunable mechanical properties. The material parameter crucial for kirigami is the Foppl-von Karman number (7, 8) γ: an indication of the ratio between in-plane stiffness and out-of-plane bending stiffness, with high numbers corresponding to membranes that more easily bend and crumple than they stretch and shear. To determine γ, we measure the bending stiffness of graphene monolayers that are 10-100 micrometres in size and obtain a value that is thousands of times higher than the predicted atomic-scale bending stiffness. Interferometric imaging attributes this finding to ripples in the membrane (9-13) that stiffen the graphene sheets considerably, to the extent that γ is comparable to that of a standard piece of paper. We may therefore apply ideas from kirigami to graphene sheets to build mechanical metamaterials such as stretchable electrodes, springs, and hinges. These results establish graphene kirigami as a simple yet powerful and customizable approach for fashioning one-atom-thick graphene sheets into resilient and movable parts with microscale dimensions., Devices such as those shown in Fig. 1a, c, d are made from polycrystalline monolayer graphene that is grown on copper by chemical vapour deposition (14), and then transferred to [...]

Carbon dioxide electroreduction facilitates the sustainable synthesis of fuels and chemicals1. Although Cu enables CO2-to-multicarbon product (C2+) conversion, the nature of the active sites under operating conditions remains elusive2. Importantly, identifying active sites of high-performance Cu nanocatalysts necessitates nanoscale, time-resolved operando techniques3–5. Here, we present a comprehensive investigation of the structural dynamics during the life cycle of Cu nanocatalysts. A 7 nm Cu nanoparticle ensemble evolves into metallic Cu nanograins during electrolysis before complete oxidation to single-crystal Cu2O nanocubes following post-electrolysis air exposure. Operando analytical and four-dimensional electrochemical liquid-cell scanning transmission electron microscopy shows the presence of metallic Cu nanograins under CO2 reduction conditions. Correlated high-energy-resolution time-resolved X-ray spectroscopy suggests that metallic Cu, rich in nanograin boundaries, supports undercoordinated active sites for C–C coupling. Quantitative structure–activity correlation shows that a higher fraction of metallic Cu nanograins leads to higher C2+ selectivity. A 7 nm Cu nanoparticle ensemble, with a unity fraction of active Cu nanograins, exhibits sixfold higher C2+ selectivity than the 18 nm counterpart with one-third of active Cu nanograins. The correlation of multimodal operando techniques serves as a powerful platform to advance our fundamental understanding of the complex structural evolution of nanocatalysts under electrochemical conditions.By investigation of structural dynamics during the life cycle of Cu nanocatalysts, correlation of multimodal operando techniques was found to serve as a powerful platform to advance understanding of their complex structural evolution. [ABSTRACT FROM AUTHOR]

The miniaturization and integration of frequency-agile microwave circuits--relevant to electronically tunable filters, antennas, resonators and phase shifters--with microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at gigahertz frequencies can be tuned by applying a quasi-static electric field (1). Appropriate systems such as [Ba.sub.x][Sr.sub.1-x]Ti[O.sub.3] have a paraelectric-ferroelectric transition just below ambient temperature, providing high tunability (1-3). Unfortunately, such films suffer significant losses arising from defects. Recognizing that progress is stymied by dielectric loss, we start with a system with exceptionally low loss--[Sr.sub.n+1][Ti.sub.n][O.sub.3n+1] phases (4,5)--in which [(SrO).sub.2] crystallographic shear (6,7) planes provide an alternative to the formation of point defects for accommodating non-stoichiometry (8,9). Here we report the experimental realization of a highly tunable ground state arising from the emergence of a local ferroelectric instability (10) in bi-axially strained [Sr.sub.n+1][Ti.sub.n][O.sub.3n+1] phases with n [greater than or equal to] 3 at frequencies up to 125 GHz. In contrast to traditional methods of modifying ferroelectrics--doping (1-3,11,12) or strain (13-16)--in this unique system an increase in the separation between the [(SrO).sub.2] planes, which can be achieved by changing n, bolsters the local ferroelectric instability. This new control parameter, n, can be exploited to achieve a figure of merit at room temperature that rivals all known tunable microwave dielectrics (3)., Ferroelectric thin films possessing a nonlinear dielectric response to a quasi-static electric field have been widely pursued for tunable dielectric devices (17-20) that work at gigahertz frequencies. [Ba.sub.x][Sr.sub.1-x]Ti[O.sub.3] is the [...]

Precise spatial control over the electrical properties of thin films is the key capability enabling the production of modern integrated circuitry. Although recent advances in chemical vapour deposition methods have enabled the large-scale production of both intrinsic and doped graphene (1-6), as well as hexagonal boron nitride (h-BN) (7-10), controlled fabrication of lateral heterostructures in these truly atomically thin systems has not been achieved. Graphene/h-BN interfaces are of particular interest, because it is known that areas of different atomic compositions may coexist within continuous atomically thin films (5,10) and that, with proper control, the bandgap and magnetic properties can be precisely engineered (11-13). However, previously reported approaches for controlling these interfaces have fundamental limitations and cannot be easily integrated with conventional lithography (14-16) Here we report a versatile and scalable process, which we call 'patterned regrowth', that allows for the spatially controlled synthesis of lateral junctions between electrically conductive graphene and insulating h-BN, as well as between intrinsic and substitutionally doped graphene. We demonstrate that the resulting films form mechanically continuous sheets across these heterojunctions. Conductance measurements confirm laterally insulating behaviour for h-BN regions, while the electrical behaviour of both doped and undoped graphene sheets maintain excellent properties, with low sheet resistances and high carrier mobilities. Our results represent an important step towards developing atomically thin integrated circuitry and enable the fabrication of electrically isolated active and passive elements embedded in continuous, one-atom-thick sheets, which could be manipulated and stacked to form complex devices at the ultimate thickness limit., Figure 1a summarizes our approach for the patterned regrowth and characterization. After growing the first film of graphene ([G.sup.1], superscript representing the growth order), we then deposit a protective layer [...]

The properties of polycrystalline materials are often dominated by the size of their grains and by the atomic structure of their grain boundaries. These effects should be especially pronounced in two-dimensional materials, where even a line defect can divide and disrupt a crystal. These issues take on practical significance in graphene, which is a hexagonal, two-dimensional crystal of carbon atoms. Single-atom-thick graphene sheets can now be produced by chemical vapour deposition (1-3) on scales of up to metres (4), making their polycrystallinity almost unavoidable. Theoretically, graphene grain boundaries are predicted to have distinct electronic (5-8), magnetic (9), chemical (10) and mechanical (11-13) properties that strongly depend on their atomic arrangement. Yet because of the five-order-of-magnitude size difference between grains and the atoms at grain boundaries, few experiments have fully explored the graphene grain structure. Here we use a combination of old and new transmission electron microscopy techniques to bridge these length scales. Using atomic-resolution imaging, we determine the location and identity of every atom at a grain boundary and find that different grains stitch together predominantly through pentagon-heptagon pairs. Rather than individually imaging the several billion atoms in each grain, we use diffraction-filtered imaging (14) to rapidly map the location, orientation and shape of several hundred grains and boundaries, where only a handful have been previously reported (15-19). The resulting images reveal an unexpectedly small and intricate patchwork of grains connected by tilt boundaries. By correlating grain imaging with scanning probe and transport measurements, we show that these grain boundaries severely weaken the mechanical strength of graphene membranes but do not as drastically alter their electrical properties. These techniques open a new window for studies on the structure, properties and control of grains and grain boundaries in graphene and other two-dimensional materials., Figure 1a shows a large array of the suspended, single-layer graphene membranes used in this study. We grew predominately single-layer graphene films on copper foils by chemical vapour deposition (1) [...]

Ferroelectric ferromagnets are exceedingly rare, fundamentally interesting multiferroic materials that could give rise to new technologies in which the low power and high speed of field-effect electronics are combined with the permanence and routability of voltage-controlled ferromagnetism (1,2). Furthermore, the properties of the few compounds that simultaneously exhibit these phenomena (1-5) are insignificant in comparison with those of useful ferroelectrics or ferromagnets: their spontaneous polarizations or magnetizations are smaller by a factor of 1,000 or more. The same holds for magnetic- or electric-field-induced multiferroics (6-8). Owing to the weak properties of single-phase multiferroics, composite and multilayer approaches involving strain-coupled piezoelectric and magnetostrictive components are the closest to application today (1,2). Recently, however, a new route to ferroelectric ferromagnets was proposed (9) by which magnetically ordered insulators that are neither ferroelectric nor ferromagnetic are transformed into ferroelectric ferromagnets using a single control parameter, strain. The system targeted, EuTi[O.sub.3], was predicted to exhibit strong ferromagnetism (spontaneous magnetization, ~ (7) Bohr magnetons per Eu) and strong ferroelectricity (spontaneous polarization, ~ 10 µC [cm.sup.-2]) simultaneously under large biaxial compressive strain (9). These values are orders of magnitude higher than those of any known ferroelectric ferromagnet and rival the best materials that are solely ferroelectric or ferromagnetic. Hindered by the absence of an appropriate substrate to provide the desired compression we turned to tensile strain. Here we show both experimentally and theoretically the emergence of a multiferroic state under biaxial tension with the unexpected benefit that even lower strains are required, thereby allowing thicker high-quality crystalline films. This realization of a strong ferromagnetic ferroelectric points the way to high-temperature manifestations of this spin-lattice coupling mechanism (10). Our work demonstrates that a single experimental parameter, strain, simultaneously controls multiple order parameters and is a viable alternative tuning parameter to composition11 for creating multiferroics., Using epitaxy and the mis-fit strain imposed by an underlying substrate, it is possible to strain dielectric thin films to per cent levels--far beyond where they would crack in bulk. [...]

Author(s): David A. Muller (corresponding author) [1, 2]; Naoyuki Nakagawa [1, 3]; Akira Ohtomo [1, 4]; John L. Grazul [1, 2]; Harold Y. Hwang [1, 3, 5] At the heart [...]

Author(s): Markus Gross [1]; David C. Müller [1]; Heinz-Georg Nothofer [2]; Ulrich Scherf [2]; Dieter Neher [3]; Christoph Bräuchle [1]; Klaus Meerholz [1] Organic light-emitting diodes (OLEDs) represent a promising [...]

Transmission electron energy-loss spectroscopy facilitated the study of the characteristic features of sp2 and sp3 carbon bonding by recording its electron density map at a spatial resolution less than one nanometre. Detailed information on the sp2 and sp3 carbon aids the understanding of the mechanism underlying diamond nucleation on an amorphous carbon layer. Results of and data obtained during the analysis of the scanning transmission electron microscope image of diamond deposited by chemical vapor deposition corroborate the occurrence of a transition from sp2 to sp3, within a region of one nanometre, during diamond nucleation.

This Letter determined that EuTi[O.sub.3], when appropriately strained, becomes a strong ferroelectric ferromagnet, in agreement with prediction. Strong ferroelectrics are proper ferroelectrics, having polarization as their order parameter, with high [...]

Ferroelectric ferromagnets are exceedingly rare, fundamentally interesting multiferroic materials that could give rise to new technologies in which the low power and high speed of field-effect electronics are combined with the permanence and routability of voltage-controlled ferromagnetism. Furthermore, the properties of the few compounds that simultaneously exhibit these phenomena are insignificant in comparison with those of useful ferroelectrics or ferromagnets: their spontaneous polarizations or magnetizations are smaller by a factor of 1,000 or more. The same holds for magnetic- or electric-field-induced multiferroics. Owing to the weak properties of single-phase multiferroics, composite and multilayer approaches involving strain-coupled piezoelectric and magnetostrictive components are the closest to application today. Recently, however, a new route to ferroelectric ferromagnets was proposed by which magnetically ordered insulators that are neither ferroelectric nor ferromagnetic are transformed into ferroelectric ferromagnets using a single control parameter, strain. The system targeted, EuTiO3, was predicted to exhibit strong ferromagnetism (spontaneous magnetization, ∼7 Bohr magnetons per Eu) and strong ferroelectricity (spontaneous polarization, ∼10 µC cm−2) simultaneously under large biaxial compressive strain. These values are orders of magnitude higher than those of any known ferroelectric ferromagnet and rival the best materials that are solely ferroelectric or ferromagnetic. Hindered by the absence of an appropriate substrate to provide the desired compression we turned to tensile strain. Here we show both experimentally and theoretically the emergence of a multiferroic state under biaxial tension with the unexpected benefit that even lower strains are required, thereby allowing thicker high-quality crystalline films. This realization of a strong ferromagnetic ferroelectric points the way to high-temperature manifestations of this spin–lattice coupling mechanism. Our work demonstrates that a single experimental parameter, strain, simultaneously controls multiple order parameters and is a viable alternative tuning parameter to composition for creating multiferroics. [ABSTRACT FROM AUTHOR]

At the heart of modern oxide chemistry lies the recognition that beneficial (as well as deleterious) materials properties can be obtained by deliberate deviations of oxygen atom occupancy from the ideal stoichiometry. Conversely, the capability to control and confine oxygen vacancies will be important to realize the full potential of perovskite ferroelectric materials, varistors and field-effect devices. In transition metal oxides, oxygen vacancies are generally electron donors, and in strontium titanate (SrTiO3) thin films, oxygen vacancies (unlike impurity dopants) are particularly important because they tend to retain high carrier mobilities, even at high carrier densities. Here we report the successful fabrication, using a pulsed laser deposition technique, of SrTiO3 superlattice films with oxygen doping profiles that exhibit subnanometre abruptness. We profile the vacancy concentrations on an atomic scale using annular-dark-field electron microscopy and core-level spectroscopy, and demonstrate absolute detection sensitivities of one to four oxygen vacancies. Our findings open a pathway to the microscopic study of individual vacancies and their clustering, not only in oxides, but in crystalline materials more generally. [ABSTRACT FROM AUTHOR]

At the heart of modern oxide chemistry lies the recognition that beneficial (as well as deleterious) materials properties can be obtained by deliberate deviations of oxygen atom occupancy from the ideal stoichiometry. Conversely, the capability to control and confine oxygen vacancies will be important to realize the full potential of perovskite ferroelectric materials, varistors and field-effect devices. In transition metal oxides, oxygen vacancies are generally electron donors, and in strontium titanate (SrTiO3) thin films, oxygen vacancies (unlike impurity dopants) are particularly important because they tend to retain high carrier mobilities, even at high carrier densities. Here we report the successful fabrication, using a pulsed laser deposition technique, of SrTiO3 superlattice films with oxygen doping profiles that exhibit subnanometre abruptness. We profile the vacancy concentrations on an atomic scale using annular-dark-field electron microscopy and core-level spectroscopy, and demonstrate absolute detection sensitivities of one to four oxygen vacancies. Our findings open a pathway to the microscopic study of individual vacancies and their clustering, not only in oxides, but in crystalline materials more generally. [ABSTRACT FROM AUTHOR]

Demonstrates that by increasing the doping level of polymer layers in organic light-emitting diodes (OLEDs), the barrier to hole injection can be continuously reduced. Findings which suggest that it is possible to significantly reduce the operating voltage of such devices while improving the light output and efficiency; Explanation of the three kinds of hole-injecting anodes employed in the devices; Use of electrochemistry for screening purposes.

Carbon dioxide electroreduction facilitates the sustainable synthesis of fuels and chemicals 1 . Although Cu enables CO 2 -to-multicarbon product (C 2+ ) conversion, the nature of the active sites under operating conditions remains elusive 2 . Importantly, identifying active sites of high-performance Cu nanocatalysts necessitates nanoscale, time-resolved operando techniques 3-5 . Here, we present a comprehensive investigation of the structural dynamics during the life cycle of Cu nanocatalysts. A 7 nm Cu nanoparticle ensemble evolves into metallic Cu nanograins during electrolysis before complete oxidation to single-crystal Cu 2 O nanocubes following post-electrolysis air exposure. Operando analytical and four-dimensional electrochemical liquid-cell scanning transmission electron microscopy shows the presence of metallic Cu nanograins under CO 2 reduction conditions. Correlated high-energy-resolution time-resolved X-ray spectroscopy suggests that metallic Cu, rich in nanograin boundaries, supports undercoordinated active sites for C-C coupling. Quantitative structure-activity correlation shows that a higher fraction of metallic Cu nanograins leads to higher C 2+ selectivity. A 7 nm Cu nanoparticle ensemble, with a unity fraction of active Cu nanograins, exhibits sixfold higher C 2+ selectivity than the 18 nm counterpart with one-third of active Cu nanograins. The correlation of multimodal operando techniques serves as a powerful platform to advance our fundamental understanding of the complex structural evolution of nanocatalysts under electrochemical conditions., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)