Nanoscale imaging of phonon dynamics by electron microscopy.

Spatially resolved vibrational mapping of nanostructures is indispensable to the development and understanding of thermal nanodevices1, modulation of thermal transport2 and novel nanostructured thermoelectric materials3-5. Through the engineering of complex structures, such as alloys, nanostructures and superlattice interfaces, one can significantly alter the propagation of phonons and suppress material thermal conductivity while maintaining electrical conductivity2. There have been no correlative experiments that spatially track the modulation of phonon properties in and around nanostructures due to spatial resolution limitations of conventional optical phonon detection techniques. Here we demonstrate two-dimensional spatial mapping of phonons in a single silicon-germanium (SiGe) quantum dot (QD) using monochromated electron energy loss spectroscopy in the transmission electron microscope. Tracking the variation of the Si optical mode in and around the QD, we observe the nanoscale modification of the composition-induced red shift. We observe non-equilibrium phonons that only exist near the interface and, furthermore, develop a novel technique to differentially map phonon momenta, providing direct evidence that the interplay between diffuse and specular reflection largely depends on the detailed atomistic structure: a major advancement in the field. Our work unveils the non-equilibrium phonon dynamics at nanoscale interfaces and can be used to study actual nanodevices and aid in the understanding of heat dissipation near nanoscale hotspots, which is crucial for future high-performance nanoelectronics.

Single-defect phonons imaged by electron microscopy.

Crystal defects affect the thermal and heat-transport properties of materials by scattering phonons and modifying phonon spectra1-8. To appreciate how imperfections in solids influence thermal conductivity and diffusivity, it is thus essential to understand phonon-defect interactions. Sophisticated theories are available to explore such interactions, but experimental validation is limited because most phonon-detecting spectroscopic methods do not reach the high spatial resolution needed to resolve local vibrational spectra near individual defects. Here we demonstrate that space- and angle-resolved vibrational spectroscopy in a transmission electron microscope makes it possible to map the vibrational spectra of individual crystal defects. We detect a red shift of several millielectronvolts in the energy of acoustic vibration modes near a single stacking fault in cubic silicon carbide, together with substantial changes in their intensity, and find that these changes are confined to within a few nanometres of the stacking fault. These observations illustrate that the capabilities of a state-of-the-art transmission electron microscope open the door to the direct mapping of phonon propagation around defects, which is expected to provide useful guidance for engineering the thermal properties of materials.

Exceptional electronic transport and quantum oscillations in thin bismuth crystals grown inside van der Waals materials.

Confining materials to two-dimensional forms changes the behaviour of the electrons and enables the creation of new devices. However, most materials are challenging to produce as uniform, thin crystals. Here we present a synthesis approach where thin crystals are grown in a nanoscale mould defined by atomically flat van der Waals (vdW) materials. By heating and compressing bismuth in a vdW mould made of hexagonal boron nitride, we grow ultraflat bismuth crystals less than 10 nm thick. Due to quantum confinement, the bismuth bulk states are gapped, isolating intrinsic Rashba surface states for transport studies. The vdW-moulded bismuth shows exceptional electronic transport, enabling the observation of Shubnikov-de Haas quantum oscillations originating from the (111) surface state Landau levels. By measuring the gate-dependent magnetoresistance, we observe multi-carrier quantum oscillations and Landau level splitting, with features originating from both the top and bottom surfaces. Our vdW mould growth technique establishes a platform for electronic studies and control of bismuth's Rashba surface states and topological boundary modes1-3. Beyond bismuth, the vdW-moulding approach provides a low-cost way to synthesize ultrathin crystals and directly integrate them into a vdW heterostructure.

Real-space charge-density imaging with sub-ångström resolution by four-dimensional electron microscopy.

The distribution of charge density in materials dictates their chemical bonding, electronic transport, and optical and mechanical properties. Indirectly measuring the charge density of bulk materials is possible through X-ray or electron diffraction techniques by fitting their structure factors1-3, but only if the sample is perfectly homogeneous within the area illuminated by the beam. Meanwhile, scanning tunnelling microscopy and atomic force microscopy enable us to see chemical bonds, but only on the surface4-6. It remains a challenge to resolve charge density in nanostructures and functional materials with imperfect crystalline structures-such as those with defects, interfaces or boundaries at which new physics emerges. Here we describe the development of a real-space imaging technique that can directly map the local charge density of crystalline materials with sub-ångström resolution, using scanning transmission electron microscopy alongside an angle-resolved pixellated fast-electron detector. Using this technique, we image the interfacial charge distribution and ferroelectric polarization in a SrTiO3/BiFeO3 heterojunction in four dimensions, and discover charge accumulation at the interface that is induced by the penetration of the polarization field of BiFeO3. We validate this finding through side-by-side comparison with density functional theory calculations. Our charge-density imaging method advances electron microscopy from detecting atoms to imaging electron distributions, providing a new way of studying local bonding in crystalline solids.

Persistent Chirality-Induced Spin-Selectivity Effect in Circular Helix Molecules.

The spin-orbit coupling (SOC), the dynamics of the nonequilibrium transport process, and the breaking of time-reversal and space-inversion symmetries have been regarded as key factors for the emergence of chirality-induced spin selectivity (CISS) and chirality-dependent spin currents in helix molecules. In this work, we demonstrated the generation of persistent CISS currents in various circular single-stranded DNAs and 310-helix proteins for the first time, regardless of whether an external magnetic flux is applied or not. This new CISS effect presents only in equilibrium transport processes, distinct from the traditional CISS observed in nonequilibrium transport processes and linear helix molecules; we term it as the PCISS effect. Notably, PCISS manifests irrespective of whether the SOC is chirality-driven or stems from heavy-metal substrates, making it an efficient way to generate chirality-locked pure spin currents. Our research establishes a novel paradigm for examining the underlying physics of the CISS effect.

Switching Intrinsic Magnetic Skyrmions with Controllable Magnetic Anisotropy in van der Waals Multiferroic Heterostructures.

Magnetic skyrmions, topologically nontrivial whirling spin textures at nanometer scales, have emerged as potential information carriers for spintronic devices. The ability to efficiently create and erase magnetic skyrmions is vital yet challenging for such applications. Based on first-principles studies, we find that switching between intrinsic magnetic skyrmion and high-temperature ferromagnetic states can be achieved in the two-dimensional van der Waals (vdW) multiferroic heterostructure CrSeI/In2Te3 by reversing the ferroelectric polarization of In2Te3. The core mechanism of this switching is traced to the controllable magnetic anisotropy of CrSeI influenced by the ferroelectric polarization of In2Te3. We propose a useful descriptor linking the presence of magnetic skyrmions to magnetic parameters and validate this connection through studies of a variety of similar vdW multiferroic heterostructures. Our work demonstrates that manipulating magnetic skyrmions via tunable magnetic anisotropies in vdW multiferroic heterostructures represents a highly promising and energy-efficient strategy for the future development of spintronics.

Origin of Interfacial Orbital Reconstruction in Perovskite Superlattices.

The competition between on-site electronic correlation and local crystal field stands out as a captivating topic in research. However, its physical ramifications often get overshadowed by influences of strong periodic potential and orbital hybridization. The present study reveals this competition may become more pronounced or even dominant in two-dimensional systems, driven by the combined effects of dimensional confinement and orbital anisotropy. This leads to electronic orbital reconstruction in certain perovskite superlattices or thin films. To explore the emerging physics, we investigate the interfacial orbital disorder-order transition with an effective Hamiltonian and how to modulate this transition through strains.

Electrically Tunable Topological Phase Transition in van der Waals Heterostructures.

The realization and control of the quantum anomalous Hall (QAH) effect are highly desirable for the development of spintronic and quantum devices. In this work, we propose a van der Waals (vdW) heterostructure of ultrathin MnBi2Se4 and Bi2Se3 layers and demonstrate that it is an excellent tunable QAH platform by using model Hamiltonian and density functional theory simulations. Its band gap closes and reopens as external electric field increases, manifesting a novel topological phase transition with an electric field of ∼0.06 V/Å. This heterostructure has other major advantageous, such as large topological band gap, perpendicular magnetization, and strong ferromagnetic ordering. Our work provides clear physical insights and suggests a new strategy for experimental realization and control of the QAH effect in real materials.

Dislocation-Assisted Quasi-Two-Dimensional Semiconducting Nanochannels Embedded in Perovskite Thin Films.

Defect engineering in perovskite thin films has attracted extensive attention recently due to the films' atomic-scale modification, allowing for remarkable flexibility to design novel nanostructures for next generation nanodevices. However, the defect-assisted three-dimensional nanostructures in thin film matrices usually has large misfit strain and thus causes unstable thin film structures. In contrast, defect-assisted one- or two-dimensional nanostructures embedded in thin films can sustain large misfit strains without relaxation, which make them suitable for defect engineering in perovskite thin films. Here, we reported the fabrication and characterization of edge-type misfit dislocation-assisted two-dimensional BiMnOx nanochannels embedded in SrTiO3/La0.7Sr0.3MnO3/TbScO3 perovskite thin films. The nanochannels are epitaxially grown from the surrounding films without noticeable misfit strain. Diode-like current rectification was spatially observed at nanochannels due to the formation of Schottky junctions between BiMnOx nanochannels and conducting La0.7Sr0.3MnO3 thin films. Such atomically scaled heterostructures constitute more flexible ultimate functional units for nanoscale electronic devices.

Curvature-Induced One-Dimensional Phonon Polaritons at Edges of Folded Boron Nitride Sheets.

Generation and manipulation of phonon polaritons are of paramount importance for understanding the interaction between an electromagnetic field and dielectric materials and furthering their application in mid-infrared optical communication. However, the formation of tunable one-dimensional phonon polaritons has been rarely realized in van der Waals layered structures. Here we report the discovery of curvature-induced phonon polaritons localized at the crease of folded hexagonal boron nitrides (h-BNs) with a few atomic layers using monochromated electron energy-loss spectroscopy. Compared to bulk regions, the creased-localized signals undergo an abnormal blue-shift of 1.4 meV. First-principles calculations reveal that the energy shift arises from the optical phonon hardening in the curled region. Interestingly, the curvature-induced phonon polariton can also be controllably achieved via an electron-beam etching approach. This work opens an avenue of tailoring local electromagnetic response and creating unique phonon polariton modes in van der Waals layered materials for diverse applications.

Confinement-Induced Catalytic Dissociation of Hydrogen Molecules in a Scanning Tunneling Microscope.

The catalytic scission of single chemical bonds has been induced by the nanoscale confinement in a scanning tunneling microscope (STM) junction. Individual hydrogen molecules sandwiched between the STM tip and a copper substrate can be dissociated solely by the reciprocating movement of the tip. The reaction rate depends sensitively on the local molecular environment, as exemplified by the effects of a nearby carbon monoxide molecule or a gold adatom. Detailed mechanisms and the nature of the transition states are revealed by density functional theory (DFT) calculations. This work provides insights into chemical reactions at the atomic scale induced by localized confinement applied by the STM tip. Furthermore, a single diatomic molecule can act as a molecular catalyst to enhance the reaction rate on a surface.

Origins of Slow Magnetic Relaxation in Single-Molecule Magnets.

Exponential and power law temperature dependences are widely used to fit experimental data of magnetic relaxation time in single molecular magnets. We derived a theory to show how these rules arise from the underling relaxation mechanisms and to clarify the conditions for their occurrence. The theory solves the puzzle of lower-than-expected Orbach barriers found in recent experiments, and elucidates it as a result of the Raman process in disguise. Our results highlight the importance of reducing the rate of direct tunneling between the ground state doublet so as to achieve longtime coherence in magnetic molecules. To this end, large spin and small transverse magnetic anisotropy can reduce magnitude of the transition operator, and rigid ligands may weaken the spin-phonon coupling in that they raise the energy of vibrational modes and better screen the acoustic phonons.

Understanding the Activity of Single-Atom Catalysis from Frontier Orbitals.

The d-band center and charge states are often used to analyze the catalytic activity of noble or transition metal surfaces and clusters, but their applicability for single-atom catalysts (SACs) is unsure. This work suggests that the spatial structure and orientation of frontier orbitals which are closest to the Fermi level of SACs play a vital role. Taking adsorption of several molecules and CO oxidization on C_{3}N-supported single-atom Au as examples, we demonstrate that adsorption and catalytic activities are well correlated with the characteristics of frontier orbitals. This work provides an effective guidance for understanding the performance of single-atom catalysts.

Density functional study of relaxation of adsorbate vibration modes: Dominance of anharmonic interaction.

Formulation and density functional workflow for calculating the lifetime of vibrational modes of molecular adsorbates on solid surfaces due to vibration-phonon coupling are presented. The anharmonic coupling is invoked to give the correct description of the origin of temperature dependence. Using pyrrolidine (C4H9N) absorbed on the Cu(001) surface as a concrete example, we show that the anharmonic coupling can be one to two orders more significant than the harmonic interaction for the broadening of vibrational spectra, especially as temperature increases. These results challenge the common assumption that the anharmonic interaction is weak and call for attention of considering its effect in quantum relaxation and related problems.

Axion Insulator State in a Ferromagnet/Topological Insulator/Antiferromagnet Heterostructure.

We propose the use of ferromagnetic insulator MnBi2Se4/Bi2Se3/antiferromagnetic insulator Mn2Bi2Se5 heterostructures for the realization of the axion insulator state. Importantly, the axion insulator state in such heterostructures only depends on the magnetization of the ferromagnetic insulator and, hence, can be observed in a wide range of external magnetic fields. Using density functional calculations and model Hamiltonian simulations, we find that the top and bottom surfaces have opposite half-quantum Hall conductances, [Formula: see text] and [Formula: see text], with a sizable global spin gap of 5.1 meV opened for the topological surface states of Bi2Se3. Our work provides a new strategy for the search of axion insulators by using van der Waals antiferromagnetic insulators along with three-dimensional topological insulators.

Unexpected Strong Thermally Induced Phonon Energy Shift for Mapping Local Temperature.

Measuring temperature in nanoscale is crucial for the research and development of microelectronic devices. Plasmon resonance has been utilized to map local temperature gradient in metallic materials (Al) due to their large coefficients of thermal expansion. However, most semiconductors (including Si and SiC) possess much smaller coefficients of thermal expansion due to their strong covalent bonding in crystal structure, for which the plasmon-based temperature measurement becomes unreliable. Here, we report an unexpected strong, thermally induced phonon energy shift in SiC by spatially resolved vibrational spectroscopy in transmission electron microscopy with in situ heating, demonstrating that this shift can be applied as a useful tool for measuring nanoscale temperature. When a bulk phonon spectrum is used, the spatial resolution of vibrational spectroscopy can be as high as one nanometer. Molecular dynamics simulations reveal that lattice expansion only contributes a small fraction of phonon energy shift and that vibrant motions away from the bonds are predominate factors. This study gains deeper insight into the understanding of dynamic behaviors of the phonon and provides a new avenue to measure local temperature in nanodevices.

Probing Magnetism in Insulating Cr2Ge2Te6 by Induced Anomalous Hall Effect in Pt.

Two-dimensional ferromagnet Cr2Ge2Te6 (CGT) is so resistive below its Curie temperature that probing its magnetism by electrical transport becomes extremely difficult. By forming heterostructures with Pt, however, we observe clear anomalous Hall effect (AHE) in 5 nm thick Pt deposited on thin (<50 nm) exfoliated flakes of CGT. The AHE hysteresis loops persist to ∼60 K, which matches well to the Curie temperature of CGT obtained from the bulk magnetization measurements. The slanted AHE loops with a narrow opening indicate magnetic domain formation, which is confirmed by low-temperature magnetic force microscopy (MFM) imaging. These results clearly demonstrate that CGT imprints its magnetization in the AHE signal of the Pt layer. Density functional theory calculations of CGT/Pt heterostructures suggest that the induced ferromagnetism in Pt may be primarily responsible for the observed AHE. Our results establish a powerful way of investigating magnetism in 2D insulating ferromagnets, which can potentially work for monolayer devices.

Bond-Selected Photodissociation of Single Molecules Adsorbed on Metal Surfaces.

We report the photoassisted activation of selected C─H bonds in individual molecules adsorbed on metal surfaces within the junction of a scanning tunneling microscope. Photons can couple to the C─H bond activation of specific hydrocarbons through a resonant photoassisted tunneling process. The molecule to be activated can be selected by positioning the tip with subangstrom resolution. Furthermore, structural tomography of the molecule and its dissociation products are imaged at different heights by the inelastic tunneling probe. The demonstration of single bond dissociation induced by resonant photoassisted tunneling electrons implies the attainment of atomic scale spatial resolution for bond-selected photochemistry.

Visualization of Nanoplasmonic Coupling to Molecular Orbital in Light Emission Induced by Tunneling Electrons.

The coupling between localized plasmon and molecular orbital in the light emission from a metallic nanocavity has been directly detected and imaged with sub-0.1 nm resolution. The light emission intensity was enhanced when the energy difference between the tunneling electrons and the lowest unoccupied molecular orbital (LUMO) of an azulene molecule matches the energy of a plasmon mode of the nanocavity defined by the Ag-tip and Ag (110) substrate of a scanning tunneling microscope (STM). The spatially resolved image of the light emission intensity matches the spatial distribution of the LUMO obtained by scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations. Our results highlight the near-field coupling of a molecular orbital to the radiative decay of a plasmonic excitation in a confined nanoscale junction.

Direct in Situ Observation and Analysis of the Formation of Palladium Nanocrystals with High-Index Facets.

Synthesizing concave-structured nanoparticles (NP) with high-index surfaces offers a viable method to significantly enhance the catalytic activity of NPs. Current approaches for fabricating concave NPs, however, are limited. Exploring novel synthesis methods requires a thorough understanding of the competing mechanisms that contribute to the evolution of surface structures during NP growth. Here, by tracking the evolution of Pd nanocubes into concave NPs at atomic scale using in situ liquid cell transmission electron microscopy, our study reveals that concave-structured Pd NPs can be formed by the cointroduction of surface capping agents and halogen ions. These two chemicals jointly create a new surface energy landscape of Pd NPs, leading to the morphological transformation. In particular, Pd atoms dissociate from the {100} surfaces with the aid of Cl- ions and preferentially redeposit to the corners and edges of the nanocubes when the capping agent polyvinylpyrrolidone is introduced, resulting in the formation of concave Pd nanocubes with distinctive high-index facets. Our work not only demonstrates a potential route for synthesizing NPs with well-defined high-index facets but also reveals the detailed atomic-scale kinetics during their formation, providing insight for future predictive synthesis.