Measuring phonon dispersion at an interface.

The breakdown of translational symmetry at heterointerfaces leads to the emergence of new phonon modes localized at the interface1. These modes have an essential role in thermal and electrical transport properties in devices, especially in miniature ones wherein the interface may dominate the entire response of the device2. Although related theoretical work began decades ago1,3-5, experimental research is totally absent owing to challenges in achieving the combined spatial, momentum and spectral resolutions required to probe localized modes. Here, using the four-dimensional electron energy-loss spectroscopy technique, we directly measure both the local vibrational spectra and the interface phonon dispersion relation for an epitaxial cubic boron nitride/diamond heterointerface. In addition to bulk phonon modes, we observe modes localized at the interface and modes isolated from the interface. These features appear only within approximately one nanometre around the interface. The localized modes observed here are predicted to substantially affect the interface thermal conductance and electron mobility. Our findings provide insights into lattice dynamics at heterointerfaces, and the demonstrated experimental technique should be useful in thermal management, electrical engineering and topological phononics.

Dislocation-tuned ferroelectricity and ferromagnetism of the BiFeO3/SrRuO3 interface.

Misfit dislocations at a heteroepitaxial interface produce huge strain and, thus, have a significant impact on the properties of the interface. Here, we use scanning transmission electron microscopy to demonstrate a quantitative unit-cell-by-unit-cell mapping of the lattice parameters and octahedral rotations around misfit dislocations at the BiFeO3/SrRuO3 interface. We find that huge strain field is achieved near dislocations, i.e., above 5% within the first three unit cells of the core, which is typically larger than that achieved from the regular epitaxy thin-film approach, thus significantly altering the magnitude and direction of the local ferroelectric dipole in BiFeO3 and magnetic moments in SrRuO3 near the interface. The strain field and, thus, the structural distortion can be further tuned by the dislocation type. Our atomic-scale study helps us to understand the effects of dislocations in this ferroelectricity/ferromagnetism heterostructure. Such defect engineering allows us to tune the local ferroelectric and ferromagnetic order parameters and the interface electromagnetic coupling, providing new opportunities to design nanosized electronic and spintronic devices.

Atomic-scale probing of heterointerface phonon bridges in nitride semiconductor.

Interface phonon modes that are generated by several atomic layers at the heterointerface play a major role in the interface thermal conductance for nanoscale high-power devices such as nitride-based high-electron-mobility transistors and light-emitting diodes. Here we measure the local phonon spectra across AlN/Si and AlN/Al interfaces using atomically resolved vibrational electron energy-loss spectroscopy in a scanning transmission electron microscope. At the AlN/Si interface, we observe various interface phonon modes, of which the extended and localized modes act as bridges to connect the bulk AlN modes and bulk Si modes and are expected to boost the phonon transport, thus substantially contributing to interface thermal conductance. In comparison, no such phonon bridge is observed at the AlN/Al interface, for which partially extended modes dominate the interface thermal conductivity. This work provides valuable insights into understanding the interfacial thermal transport in nitride semiconductors and useful guidance for thermal management via interface engineering.

Nanoscale Localized Phonons at Al2O3 Grain Boundaries.

Nanoscale defects like grain boundaries (GBs) would introduce local phonon modes and affect the bulk materials' thermal, electrical, optical, and mechanical properties. It is highly desirable to correlate the phonon modes and atomic arrangements for individual defects to precisely understand the structure-property relation. Here we investigated the localized phonon modes of Al2O3 GBs by combination of the vibrational electron energy loss spectroscopy (EELS) in scanning transmission electron microscope and density functional perturbation theory (DFPT). The differences between GB and bulk obtained from the vibrational EELS show that the GB exhibited more active vibration at the energy range of <50 meV and >80 meV, and further DFPT results proved the wide distribution of bond lengths at GB are the main factor for the emergence of local phonon modes. This research provides insights into the phonon-defect relation and would be of importance in the design and application of polycrystalline materials.

Tunable Interband Transitions in Twisted h-BN/Graphene Heterostructures.

In twisted h-BN/graphene heterostructures, the complex electronic properties of the fast-traveling electron gas in graphene are usually considered to be fully revealed. However, the randomly twisted heterostructures may also have unexpected transition behaviors, which may influence the device performance. Here, we study the twist-angle-dependent coupling effects of h-BN/graphene heterostructures using monochromatic electron energy loss spectroscopy. We find that the moiré potentials alter the band structure of graphene, resulting in a redshift of the intralayer transition at the M point, which becomes more pronounced up to 0.22 eV with increasing twist angle. Furthermore, the twisting of the Brillouin zone of h-BN relative to the graphene M point leads to tunable vertical transition energies in the range of 5.1-5.6 eV. Our findings indicate that twist-coupling effects of van der Waals heterostructures should be carefully considered in device fabrications, and the continuously tunable interband transitions through the twist angle can serve as a new degree of freedom to design optoelectrical devices.

Palladium-Catalyzed Thiocarbonylation of Aryl Iodides Using CO2.

The first example of catalytic thiocarbonylation of aryl iodides using CO2 has been achieved employing a combination of PdCl2 and carbazole-derived phosphine ligands. Under mild conditions, a broad scope of aryl iodides were converted to the desired thioester products in the presence of aryl or alkyl thiols (33 examples, up to 96% yields). The choice of metal, ligands, and reductant were crucial for high efficiency and chemoselectivity. Moreover, this strategy provided an effective method for the late-stage functionalization of biorelevant molecules.

Role of excited-state hydrogen bonding in CO2 photoreduction catalyzed by sodium magnesium chlorophyll.

In this paper, we report a joint experimental and computational study to elaborate the mechanism for the photocatalytic CO2 reduction reaction (CO2RR). Experimental results indicate that the catalyst (sodium magnesium chlorophyll, MgChlNa2), which has a well-defined structure for calculation and understanding, can achieve the photoreduction of CO2 to CO only using water as a dispersant, without adding any photosensitizer or sacrificial agent. Subsequently, a series of structural models of the hydrogen-bonded complexes of the catalyst were constructed and outlined via utilizing density functional theory (DFT) calculations, including photophysical and photochemical processes. The results confirm that the rate-limiting step of the whole CO2RR was the intersystem crossing process. The electron and proton transfers involved in photophysical and photochemical processes are induced by hydrogen bonds in the excited states. The combination of experiments and calculations will provide an important reference for the design of high-efficiency photocatalysts in the photocatalytic CO2RR.

Electron Microscopy Probing Electron-Photon Interactions in SiC Nanowires with Ultrawide Energy and Momentum Match.

Light-matter interactions are commonly probed by optical spectroscopy, which, however, has some fundamental limitations such as diffraction-limited spatial resolution, tiny momentum transfer, and noncontinuous excitation/detection. In this work, through the use of scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) with ultrawide energy and momentum match and subnanometer spatial resolution, the longitudinal Fabry-Perot (FP) resonating modes and the transverse whispering-gallery modes (WGMs) in individual SiC nanowires are simultaneously excited and detected, which span from near-infrared (∼1.2 µm) to ultraviolet (∼0.2 µm) spectral regime, and the momentum transfer can range up to 108 cm-1. The size effects on the resonant spectra of nanowires are also revealed. This work provides an alternative technique to optical resonating spectroscopy and light-matter interactions in dielectric nanostructures, which is promising for modulating free electrons via photonic structures.

Direct observation of highly confined phonon polaritons in suspended monolayer hexagonal boron nitride.

Phonon polaritons enable light confinement at deep subwavelength scales, with potential technological applications, such as subdiffraction imaging, sensing and engineering of spontaneous emission. However, the trade-off between the degree of confinement and the excitation efficiency of phonon polaritons prevents direct observation of these modes in monolayer hexagonal boron nitride (h-BN), where they are expected to reach ultrahigh confinement. Here, we use monochromatic electron energy-loss spectroscopy (about 7.5 meV energy resolution) in a scanning transmission electron microscope to measure phonon polaritons in monolayer h-BN, directly demonstrating the existence of these modes as the phonon Reststrahlen band (RS) disappears. We find phonon polaritons in monolayer h-BN to exhibit high confinement (>487 times smaller wavelength than that of light in free space) and ultraslow group velocity down to about 10-5c. The large momentum compensation provided by electron beams additionally allows us to excite phonon polaritons over nearly the entire RS band of multilayer h-BN. These results open up a broad range of opportunities for the engineering of metasurfaces and strongly enhanced light-matter interactions.

Erratum: Chiral Spin-Wave Velocities Induced by All-Garnet Interfacial Dzyaloshinskii-Moriya Interaction in Ultrathin Yttrium Iron Garnet Films [Phys. Rev. Lett. 124, 027203 (2020)].

This corrects the article DOI: 10.1103/PhysRevLett.124.027203.

Manganese-Catalyzed [4 + 2] Annulation of N-H Amidines with Vinylene Carbonate via C-H Activation.

Manganese-catalyzed C-H bond functionalization of aryl amidines for the synthesis of 1-aminoisoquinolines in the presence of vinylene carbonate has been developed. The reaction features a broad substrate scope and proceeds under mild reaction conditions with only the carbonate anion as the byproduct.

Chiral Spin-Wave Velocities Induced by All-Garnet Interfacial Dzyaloshinskii-Moriya Interaction in Ultrathin Yttrium Iron Garnet Films.

Spin waves can probe the Dzyaloshinskii-Moriya interaction (DMI), which gives rise to topological spin textures, such as skyrmions. However, the DMI has not yet been reported in yttrium iron garnet (YIG) with arguably the lowest damping for spin waves. In this work, we experimentally evidence the interfacial DMI in a 7-nm-thick YIG film by measuring the nonreciprocal spin-wave propagation in terms of frequency, amplitude, and most importantly group velocities using all electrical spin-wave spectroscopy. The velocities of propagating spin waves show chirality among three vectors, i.e., the film normal direction, applied field, and spin-wave wave vector. By measuring the asymmetric group velocities, we extract a DMI constant of 16 µJ/m^{2}, which we independently confirm by Brillouin light scattering. Thickness-dependent measurements reveal that the DMI originates from the oxide interface between the YIG and garnet substrate. The interfacial DMI discovered in the ultrathin YIG films is of key importance for functional chiral magnonics as ultralow spin-wave damping can be achieved.

Zn-Catalyzed Cyanation of Aryl Iodides.

We report the first example of zinc-catalyzed cyanation of aryl iodides with formamide as the cyanogen source. The transformation was promoted by the bisphosphine Nixantphos ligand. Under optimized conditions, a variety of electron-donating and electron-withdrawing aryl iodides were converted into nitrile products in good to excellent yields. This approach is an exceedingly simple and benign method for the synthesis of aryl nitriles and is likely to proceed via a dinuclear Zn-concerted catalysis.

Probing Far-Infrared Surface Phonon Polaritons in Semiconductor Nanostructures at Nanoscale.

Phonon polaritons hold potential prospects of nanophotonic applications at the mid- and far-infrared wavelengths. However, their experimental investigation in the far-infrared range has long been a technical challenge due to the lack of suitable light sources and detectors. To obviate these difficulties, here we use an electron probe with sub-10 meV energy resolution and subnanometer spatial resolution to study far-infrared surface phonon polaritons (∼50-70 meV) in ZnO nanostructures. We observe ultraslow propagation and interference fringes of propagating surface phonon polaritons and obtain their dispersion relation through measurements in the coordinate space. By mapping localized modes in nanowires and flakes, we reveal their localized nature and investigate geometry and size effects. Associated with simulation, we show that surface phonon polariton behaviors can be well described by the local continuum dielectric model. Our work paves the way for spatial-resolved investigation of surface phonon polaritons by electron probes and forwards polaritonics in the far-infrared range.

GaCl3-Catalyzed C-H Cyanation of Indoles with N-Cyanosuccinimide.

An efficient GaCl3-catalyzed direct cyanation of indoles and pyrroles using bench-stable electrophilic cyanating agent N-cyanosuccinimide was achieved and afforded 3-cyanoindoles and 2-cyanopyrroles in good yields and excellent regioselectivities. Notably, this protocol exhibited high reactivity for unprotected indoles and was applicable to a broad range of indole and pyrrole substrates.

A General and Selective Rhodium-Catalyzed Reduction of Amides, N-Acyl Amino Esters, and Dipeptides Using Phenylsilane.

This article describes a selective reduction of functionalized amides, including N-acyl amino esters and dipeptides, to the corresponding amines using simple [Rh(acac)(cod)]. The catalyst shows excellent chemoselectivity in the presence of different sensitive functional moieties.

Relay iron/chiral Brønsted acid catalysis: enantioselective hydrogenation of benzoxazinones.

An asymmetric hydrogenation reaction of benzoxazinones has been accomplished via a relay iron/chiral Brønsted acid catalysis. This approach provides a variety of chiral dihydrobenzoxazinones in good to high yields (75-96%) and enantioselectivities (up to 98:2 er). It is noteworthy that challenging 3-alkyl-substituted substrates underwent highly enantioselective reduction. A key to success is the utilization of a nonchiral phosphine ligand to reduce disadvantageous background reactions through tuning the catalytic activity of Fe3(CO)12.

Rh(I)-Catalyzed Hydroamidation of Olefins via Selective Activation of N-H Bonds in Aliphatic Amines.

Hydroamidation of olefins constitutes an ideal, atom-efficient method to prepare carboxylic amides from easily available olefins, CO, and amines. So far, aliphatic amines are not suitable for these transformations. Here, we present a ligand- and additive-free Rh(I) catalyst as solution to this problem. Various amides are obtained in good yields and excellent regioselectivities. Notably, chemoselective amidation of aliphatic amines takes place in the presence of aromatic amines and alcohols. Mechanistic studies reveal the presence of Rh-acyl species as crucial intermediates for the selectivity and rate-limiting step in the proposed Rh(I)-catalytic cycle.

Synthesis and Characterization of Iron-Nitrogen-Doped Graphene/Core-Shell Catalysts: Efficient Oxidative Dehydrogenation of N-Heterocycles.

An important goal for nanocatalysis is the development of flexible and efficient methods for preparing active and stable core-shell catalysts. In this respect, we present the synthesis and characterization of iron oxides surrounded by nitrogen-doped-graphene shells immobilized on carbon support (labeled FeOx@NGr-C). Active catalytic materials are obtained in a simple, scalable and two-step method via pyrolysis of iron acetate and phenanthroline and subsequent selective leaching. The optimized FeOx@NGr-C catalyst showed high activity in oxidative dehydrogenations of several N-heterocycles. The utility of this benign methodology is demonstrated by the synthesis of pharmaceutically relevant quinolines. In addition, mechanistic studies prove that the reaction progresses via superoxide radical anions (·O2(-)).

Convenient Reductive Methylation of Amines with Carbonates at Room Temperature.

Methylation of amines is a fundamental and commonly used reaction in organic synthesis. Many methods are known including various reductive methylations using formaldehyde, formic acid, or carbon dioxide in the presence of reductants. However, several of these methods suffer from limited substrate scope and chemoselectivity because of the different nucleophilicities of substrates. In this respect, the combination of carbonates and hydrosilanes is a valuable methylation source in the presence of Pt-based catalysts. This highly tunable method allows for methylation of both aromatic and aliphatic amines, and chemoselective methylation of aminoalcohols and diamines. Notably, the in situ-formed catalyst can also be used for the reduction of carbonates to methanol at room temperature. Mechanistic insights on intermediates formed during the reaction pathway were obtained by using ESI mass spectrometry.