Search

Conversion of charge to orbital angular momentum through the orbital Hall effect (OHE) holds transformative potential for the development of orbital-based electronics, however, it is challenging to directly observe the electrically generated orbital accumulation. Here, we detect the OHE by directly quantifying the orbital accumulation along the edges of a titanium thin film using a scanning transmission electron microscope. We measure the Ti L-edge using electron energy-loss spectroscopy with nanometer resolution and find a sizable orbital accumulation at the sample's outer perimeters, consistent with all signatures expected for the OHE, and determine an orbital diffusion length $\ell_o \approx 7.3$ nm. Our data points to a surprising dependence of the orbital diffusion length on the nano-structural morphology., Comment: 13 pages, 3 figures

Fluids under extreme confinement exhibit distinctly new properties compared to their bulk analogs. Understanding the structure and intermolecular bonding of confined water lays the foundation for creating and improving applications at the water-energy nexus. However, probing confined water experimentally at the length scale of intermolecular and surface forces has remained a challenge. Here, we report a combined experiment/theory framework to reveal changes in H-bonding environment and the underlying molecular structure of confined water inside individual carbon nanotubes. H-bonding is directly probed through the O-H stretch frequency with vibrational electron energy-loss spectroscopy and compared to spectra from molecular-dynamics simulations based on density-functional-theory. Experimental spectra show that water in larger carbon nanotubes exhibit the bonded O-H vibrations of bulk water, but at smaller diameters, the frequency blueshifts to near the 'free' O-H stretch found in water vapor and hydrophobic surfaces. The matching simulations reveal that, in addition to steric confinement, the tube's vibrations play a key role in breaking up the H-bond network, resulting in an orientationally-dispersed, non-H-bonded phase. Furthermore, the temperature-dependence of the vibrations is investigated, providing insights into phase transitions and the confined-water density. This research demonstrates the potential of the experiment/theory framework to explore unprecedented aspects of structure and bonding in confined fluids.

Plasmonic lattice nanostructures are of technological interest because of their capacity to manipulate light below the diffraction limit. Here, we present a detailed study of dark and bright modes in the visible and near-infrared energy regime of an inverted plasmonic honeycomb lattice by a combination of Au+ focused ion beam lithography with nanometric resolution, optical and electron spectroscopy, and finite-difference time-domain simulations. The lattice consists of slits carved in a gold thin film, exhibiting hotspots and a set of bright and dark modes. We proposed that some of the dark modes detected by electron energy-loss spectroscopy are caused by antiferroelectric arrangements of the slit polarizations with two times the size of the hexagonal unit cell. The plasmonic resonances take place within the 0.5_2 eV energy range, indicating that they could be suitable for a synergistic coupling with excitons in two-dimensional transition metal dichalcogenides materials or for designing nanoscale sensing platforms based on near-field enhancement over a metallic surface.

Vibrations in materials and nanostructures at sufficiently high temperatures result in anharmonic atomic displacements, which leads to new phenomena such as thermal expansion and multiphonon scattering processes, with a profound impact on temperature-dependent material properties including thermal conductivity, phonon lifetimes, nonradiative electronic transitions, and phase transitions. Nanoscale momentum-resolved vibrational spectroscopy, which has recently become possible on monochromated scanning-transmission-electron microscopes, is a unique method to probe the underpinnings of these phenomena. Here we report momentum-resolved vibrational spectroscopy in hexagonal boron nitride at temperatures of 300, 800, and 1300 K across three Brillouin zones (BZs) that reveals temperature-dependent phonon energy shifts and demonstrates the presence of strong Umklapp processes. Density-functional-theory calculations of temperature-dependent phonon self-energies reproduce the observed energy shifts and identify the contributing mechanisms., Comment: 21 pages, 4 figures, 2 tables, 3 supplemental figures, 3 supplemental tables

Magnon diffuse scattering (MDS) signals could be studied with high spatial resolution in scanning transmission electron microscopy (STEM), thanks to recent technological progress in electron energy loss spectroscopy. However, detecting MDS signals in STEM is challenging due to their overlap with stronger thermal diffuse scattering (TDS) signals. In bcc Fe at 300 K, MDS signals greater than or comparable to TDS signals occur under the central Bragg disk, into a currently inaccesible energy-loss region. Therefore, to detect MDS in STEM, it is necessary to find conditions in which TDS and MDS signals can be separated. Temperature may be a key factor due to the distinct thermal signatures of magnon and phonon signals. In this work, we present a study on the effects of temperature on MDS and TDS in bcc Fe -- considering a detector outside the central Bragg disk and a fixed convergent electron probe -- using the frozen phonon and frozen magnon multislice methods. Our study reveals that neglecting the effects of atomic vibrations causes the MDS signal to grow approximately linearly up to the Curie temperature of Fe, after which it exhibits less variation. The MDS signal displays an alternating behavior due to dynamical diffraction, instead of increasing monotonically as a function of thickness. Including the effects of atomic vibrations through a complex atomic electrostatic potential causes the linear growth of the MDS signal to change to a non-linear behavior that exhibits a predominant peak for a sample of thickness 16.072 nm at 1100 K. In contrast, the TDS signal grows more linearly than the MDS signal but still exhibits appreciable dynamical diffraction effects. An analysis of the signal-to-noise ratio (SNR) shows that the MDS signal can be a statistically significant contribution to the total scattering intensity under realizable measurement conditions and acquisition times., Comment: *) Changed title to "Unveiling the impact of temperature on magnon diffuse scattering detection in the transmission electron microscope." *) Incorporated the effects of absorption due to phonons in the magnon-diffuse-scattering calculations through a complex atomic electrostatic potential

The ability to control and modify infrared excitations in condensed matter is of both fundamental and application interests. Here we explore a system supporting low-energy excitations, in particular, mid-infrared localized plasmon modes and phonon polaritons that are tuned to be strongly coupled. We study the coupled modes by using far-field infrared spectroscopy, state-of-the-art monochromated electron energy-loss spectroscopy, numerical simulations and analytical modeling. We demonstrate that the electron probe facilitates a precise characterization of polaritons constituting the coupled system, and enables an active control over the coupling and the resulting sample response both in frequency and space. Our work establishes a rigorous description of the spectral features observed in light- and localized electron-based spectroscopies, which can be extended to analogous optical systems with applications in heat management, and electromagnetic field concentration or nanofocusing.

Interfaces impede heat flow in micro/nanostructured systems. Conventional theories for interfacial thermal transport were derived based on bulk phonon properties of the materials making up the interface without explicitly considering the atomistic interfacial details, which are found critical to correctly describing thermal boundary conductance (TBC). Recent theoretical studies predicted the existence of localized phonon modes at the interface which can play an important role in understanding interfacial thermal transport. However, experimental validation is still lacking. Through a combination of Raman spectroscopy and high-energy resolution electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope, we report the first experimental observation of localized interfacial phonon modes at ~12 THz at a high-quality epitaxial Si-Ge interface. These modes are further confirmed using molecular dynamics simulations with a high-fidelity neural network interatomic potential, which also yield TBC agreeing well with that measured from time-domain thermoreflectance (TDTR) experiments. Simulations find that the interfacial phonon modes have obvious contribution to the total TBC. Our findings may significantly contribute to the understanding of interfacial thermal transport physics and have impact on engineering TBC at interfaces in applications such as electronics thermal management and thermoelectric energy conversion.

We present a theory and a simulation of thermal diffuse scattering due to the excitation of magnons in scanning transmission electron microscopy. The calculations indicate that magnons can present atomic contrast when detected by electron energy-loss spectroscopy using atomic-size electron beams. The results presented here indicate that the intensity of the magnon diffuse scattering in bcc iron at 300~K is 4 orders of magnitude weaker than the intensity of thermal diffuse scattering arising from atomic vibrations. In an energy range where the phonon and magnon dispersions do not overlap, a monochromated scanning transmission electron microscope equipped with direct electron detectors for spectroscopy is expected to resolve the magnon spectral signatures., Comment: 6 pages, 3 figures

Achieving large-size two-dimensional (2D) crystals is key to fully exploiting their remarkable functionalities and application potentials. Chemical vapor deposition (CVD) growth of 2D semiconductors such as monolayer MoS2 has been reported to be activated by halide salts, yet clear identification of the underlying mechanism remains elusive. Here we provide unambiguous experimental evidence showing that the MoS2 growth dynamics are halogen-dependent through the Br{\o}nsted-Evans-Polanyi relation, based on which we build a growth model by considering MoS2 edge passivation by halogens, and theoretically reproduces the trend of our experimental observations. These mechanistic understandings enable us to further optimize the fast growth of MoS2 and reach record-large domain sizes that should facilitate practical applications., Comment: 18 pages, 5 figures

Manipulating materials with atomic-scale precision is essential for the development of next-generation material design toolbox. Tremendous efforts have been made to advance the compositional, structural, and spatial accuracy of material deposition and patterning. The family of 2D materials provides an ideal platform to realize atomic-level material architectures. The wide and rich physics of these materials have led to fabrication of heterostructures, superlattices, and twisted structures with breakthrough discoveries and applications. Here, we report a novel atomic-scale material design tool that selectively breaks and forms chemical bonds of 2D materials at room temperature, called atomic-layer substitution (ALS), through which we can substitute the top layer chalcogen atoms within the 3-atom-thick transition-metal dichalcogenides using arbitrary patterns. Flipping the layer via transfer allows us to perform the same procedure on the other side, yielding programmable in-plane multi-heterostructures with different out-of-plane crystal symmetry and electric polarization. First-principle calculations elucidate how the ALS process is overall exothermic in energy and only has a small reaction barrier, facilitating the reaction to occur at room temperature. Optical characterizations confirm the fidelity of this design approach, while TEM shows the direct evidence of Janus structure and suggests the atomic transition at the interface of designed heterostructure. Finally, transport and Kelvin probe measurements on MoXY (X,Y=S,Se; X and Y corresponding to the bottom and top layers) lateral multi-heterostructures reveal the surface potential and dipole orientation of each region, and the barrier height between them. Our approach for designing artificial 2D landscape down to a single layer of atoms can lead to unique electronic, photonic and mechanical properties previously not found in nature.

An essay about future challenges in electron microscopy, Comment: 1 Figure

In this Letter, we exploit recent breakthroughs in monochromated aberration-corrected scanning transmission electron microscopy (STEM) to resolve infrared plasmonic Fano antiresonances in individual nanofabricated disk-rod dimers. Using a combination of electron energy-loss spectroscopy (EELS) and theoretical modeling, we investigate and characterize a subspace of the weak coupling regime between quasi-discrete and quasi-continuum localized surface plasmon resonances where infrared plasmonic Fano antiresonances appear. This work illustrates the capability of STEM instrumentation to experimentally observe nanoscale plasmonic responses that were previously the domain only of higher resolution infrared spectroscopies.

Molecules intercalating two-dimensional (2D) materials form complex structures that have been mostly characterized by spatially averaged techniques. Here we use aberration-corrected scanning transmission electron microscopy and density-functional-theory (DFT) calculations to study the atomic structure of bilayer graphene (BLG) and few-layer graphene (FLG) intercalated with FeCl$_3$. In BLG we discover two distinct intercalated structures that we identify as monolayer-FeCl$_3$ and monolayer-FeCl$_2$. The two structures are separated by atomically sharp boundaries and induce large but different free-carrier densities in the graphene layers, $7.1\times10^{13}$ cm$^{-2}$ and $7.1\times10^{13}$ cm$^{-2}$ respectively. In FLG, we observe multiple FeCl$_3$ layers stacked in a variety of possible configurations with respect to one another. Finally, we find that the microscope's electron beam can convert the FeCl$_3$ monolayer into FeOCl monolayers in a rectangular lattice. These results reveal the need for a combination of atomically-resolved microscopy, spectroscopy, and DFT calculations to identify intercalated structures and study their properties.

The exceptional electronic, optical and chemical properties of two-dimensional materials strongly depend on the 3D atomic structure and crystal defects. Using Re-doped MoS2 as a model, here we develop scanning atomic electron tomography (sAET) to determine the 3D atomic positions and crystal defects such as dopants, vacancies and ripples with a precision down to 4 picometers. We measure the 3D bond distortion and local strain tensor induced by single dopants for the first time. By directly providing experimental 3D atomic coordinates to density functional theory (DFT), we obtain more truthful electronic band structures than those derived from conventional DFT calculations relying on relaxed 3D atomic models, which is confirmed by photoluminescence measurements. We anticipate that sAET is not only generally applicable to the determination of the 3D atomic coordinates of 2D materials, heterostructures and thin films, but also could transform ab initio calculations by using experimental 3D atomic coordinates as direct input to better predict and discover new physical, chemical and electronic properties., Comment: 44 pages, 18 figures

We propose a magnetic measurement method based on combining depth sectioning and electron magnetic circular dichroism in scanning transmission electron microscopy. Electron vortex beams with large convergence angles, as those achievable in current state-of-the-art aberration correctors, could produce atomic lateral resolution and depth resolution below 2~nm., Comment: 5 pages, 3 figures

Isotope labeling is a fundamental staple for the study of cellular metabolism and protein function. The conventional techniques that allow resolution and identification of isotopically-labeled biomarkers, such as mass spectrometry and infrared spectroscopy, are macroscopic in nature and have the disadvantage of requiring relatively large quantities of material and lacking spatial resolution. Here, we record the vibrational spectra of an {\alpha}-amino acid, L-alanine, using spatially-resolved monochromated electron energy loss spectroscopy (EELS) to directly resolve carbon-site-specific isotopic labels in a scanning transmission electron microscope. The EELS is acquired in aloof mode, meaning the probe is positioned away from the sample (~20 nm) sparing the sensitive biomolecule from the high-energy excitations, while the vibrational modes are investigated. An isotopic red-shift of 5.3 meV was obtained for the C=O stretching mode in the carboxylic acid group for 13C-enriched L-alanine when compared with naturally occurring 12C L-alanine, which is confirmed by macroscopic infrared spectroscopy measurements and theoretical calculations. The EELS experiments presented here are the first demonstration of non-destructive resolution and identification of isotopically-labeled amino acids in the electron microscope, opening a new door for the study of biological matter at the nanoscale.

Surface phonon polaritons (SPhPs) - the surface-bound electromagnetic modes of a polar material resulting from the coupling of light with optic phonons - offer immense technological opportunities for nanophotonics in the infrared (IR) spectral region. Here, we present a novel approach to overcome the major limitation of SPhPs, namely the narrow, material-specific spectral range where SPhPs can be supported, called the Reststrahlen band. We use an atomic-scale superlattice (SL) of two polar semiconductors, GaN and AlN, to create a hybrid material featuring layer thickness-tunable optic phonon modes. As the IR dielectric function is governed by the optic phonon behavior, such control provides a means to create a new dielectric function distinct from either constituent material and to tune the range over which SPhPs can be supported. This work offers the first glimpse of the guiding principles governing the degree to which the dielectric function can be designed using this approach.

Atomic-level structural changes in materials are important but challenging to study. Here, we demonstrate the dynamics and the possibility of manipulating a phosphorus dopant atom in graphene using scanning transmission electron microscopy (STEM). The mechanisms of various processes are explored and compared with those of other dopant species by first-principles calculations. This work paves the way for designing a more precise and optimized protocol for atomic engineering.

Understanding the near-field electromagnetic interactions that produce optical orbital angular momentum (OAM) is central to the integration of twisted light into nanotechnology. Here, we examine the cathodoluminescence (CL) of plasmonic vortices carrying OAM generated in spiral nanostructures through scanning transmission electron microscopy (STEM). The nanospiral geometry defines the photonic local density of states (LDOS) sampled by STEM-CL, which provides access to the phase and amplitude of the plasmonic vortex with nanometer spatial and meV spectral resolution. We map the full spectral dispersion of the plasmonic vortex in the spiral structure and examine the effects of increasing topological charge on the plasmon phase and amplitude in the detected CL signal. The vortex is mapped in CL over a broad spectral range, and deviations between the predicted and detected positions of near-field optical signatures of as much as 5 per cent are observed. Finally, enhanced luminescence is observed from concentric spirals of like handedness compared to that from concentric spirals of opposite handedness, indicating the potential to couple plasmonic vortices to chiral nanostructures for sensitive detection and manipulation of optical OAM.

Alloying/doping in two-dimensional material has been important due to wide range band gap tunability. Increasing the number of components would increase the degree of freedom which can provide more flexibility in tuning the band gap and also reduced the growth temperature. Here, we report synthesis of quaternary alloys MoxW1-xS2ySe2(1-y) using chemical vapour deposition. The composition of alloys has been tuned by changing the growth temperatures. As a result, we can tune the bandgap which varies from 1.73 eV to 1.84 eV. The detailed theoretical calculation supports the experimental observation and shows a possibility of wide tunability of bandgap.

This study provides an account of the bulk preparation of TlBa2Ca2Cu3O9-{\delta} (Tl-1223) superconductor at ambient pressure, and the Tc features under thermal-annealing conditions. The as-prepared Tl-1223 (Tc =106 K) presents a significantly higher Tc = 125 K after annealing the polycrystalline material in either flowing Ar+4% H2, or N2 gases. In order to understand the fundamental reasons for a particular Tc, we refined the average bulk structures using powder X-ray diffraction data. Although Ar+4%H2 annealed Tl-1223 shows an increased c lattice parameter, it is approximately unchanged upon N2-anneal. Due to such indeterminate conclusions on the average structural changes, local structures were investigated at using aberration-corrected scanning-transmission electron microscopy technique. Similar compositional changes in the atomic arrangements of both annealed-samples of Tl-1223 were detected in which the plane containing Ca atomic layer gives a non-uniform contrast, due to substitution of some heavier Tl. In this report, extensive bulk properties are summarized through temperature-dependent resistivity, and shielding and Meissner fractions of magnetic susceptibility results; the bulk and local structures are investigated to correlate to properties., Comment: Submitted to SuST; 6 figures

Iron-based superconductors (FBS) comprise several families of compounds having the same atomic building blocks for superconductivity, but large discrepancies among their physical properties. A longstanding goal in the field has been to decipher the key underlying factors controlling TC and the various doping mechanisms. In FBS materials this is complicated immensely by the different crystal and magnetic structures exhibited by the different families. In this paper, using aberration-corrected scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS), we observe a universal behavior in the hole concentration and magnetic moment across different families. All the parent materials have the same total number of electrons in the Fe 3d bands; however, the local Fe magnetic moment varies due to different orbital occupancy. Although the common understanding has been that both long-range and local magnetic moments decrease with doping, we find that, near the onset of superconductivity, the local magnetic moment increases and shows a dome-like maximum near optimal doping, where no ordered magnetic moment is present. In addition, we address a longstanding debate concerning how Co substitutions induces superconductivity in the 122 arsenide family, showing that the 3d band filling increases a function of doping. These new microscopic insights into the properties of FBS demonstrate the importance of spin fluctuations for the superconducting state, reveal changes in orbital occupancy among different families of FBS, and confirm charge doping as one of the mechanisms responsible for superconductivity in 122 arsenides.

The calculations presented here reveal that an electron probe carrying orbital angular momentum is just a particular case of a wider class of electron beams that can be used to measure electron magnetic circular dichroism (EMCD) with atomic resolution. It is possible to obtain an EMCD signal with atomic resolution by simply breaking the symmetry of the electron probe phase distribution using the aberration-corrected optics of an scanning transmission electron microscope. The required phase distribution of the probe depends on the magnetic symmetry and crystal structure of the sample. The calculations indicate that EMCD signals utilizing the phase of the electron probe are as strong as those obtained by nanodiffraction methods., Comment: 8 pages, 5 figures

We report on the first demonstration of p-type doping in large area few-layer films of (0001)-oriented chemical vapor deposited (CVD) MoS2. Niobium was found to act as an efficient acceptor up to relatively high density in MoS2 films. For a hole density of 4 x 1020 cm-3 Hall mobility of 8.5 cm2V-1s-1 was determined, which matches well with the theoretically expected values. XRD and Raman characterization indicate that the film had good out-of-plane crystalline quality. Absorption measurements showed that the doped sample had similar characteristics to high-quality undoped samples, with a clear absorption edge at 1.8 eV. This demonstration of p-doping in large area epitaxial MoS2 could help in realizing a wide variety of electrical and opto-electronic devices based on layered metal dichalcogenides.

Single layered molybdenum disulfide with a direct bandgap is a promising two-dimensional material that goes beyond graphene for next generation nanoelectronics. Here, we report the controlled vapor phase synthesis of molybdenum disulfide atomic layers and elucidate a fundamental mechanism for the nucleation, growth, and grain boundary formation in its crystalline monolayers. Furthermore, a nucleation-controlled strategy is established to systematically promote the formation of large-area single- and few-layered films. The atomic structure and morphology of the grains and their boundaries in the polycrystalline molybdenum disulfide atomic layers are examined and first-principles calculations are applied to investigate their energy landscape. The electrical properties of the atomic layers are examined and the role of grain boundaries is evaluated. The uniformity in thickness, large grain sizes, and excellent electrical performance of these materials signify the high quality and scalable synthesis of the molybdenum disulfide atomic layers., Comment: Submitted on Jan 6th 2013

We report the chemical synthesis of Fe-core/Au-shell nanoparticles by a reverse micelle method, and the investigation of their growth mechanisms and oxidation-resistant characteristics. The core-shell structure and the presence of the Fe & Au phases have been confirmed by transmission electron microscopy, energy dispersive spectroscopy, X-ray diffraction, Mossbauer spectroscopy, and inductively coupled plasma techniques. Additionally, atomic-resolution Z-contrast imaging and electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) have been used to study details of the growth processes. The Au-shell grows by nucleating on the Fe-core surface before coalescing. The magnetic moments of such nanoparticles, in the loose powder form, decrease over time due to oxidation. The less than ideal oxidation-resistance of the Au shell may have been caused by the rough Au surfaces. However, in the pressed pellet form, electrical transport measurements show that the particles are fairly stable, as the resistance of the pellet does not change appreciably over time., Comment: 21 pages, 4 figures, Chemistry of Materials, in press

We have examined polycrystalline MgB2 by electron energy loss spectroscopy (EELS) and density of state calculations. In particular, we have studied two different crystal orientations, [110] and [001] with respect to the incident electron beam direction, and found significant changes in the near-edge fine-structure of the B K-edge. Density functional theory suggests that the pre-peak of the B K-edge core loss is composed of a mixture of pxy and pz hole states and we will show that these contributions can be distinguished only with an experimental energy resolution better than 0.5 eV. For conventional TEM/STEM instruments with an energy resolution of ~1.0 eV the pre-peak still contains valuable information about the local charge carrier concentration that can be probed by core-loss EELS. By considering the scattering momentum transfer for different crystal orientations, it is possible to analytically separate pxy and pz components from of the experimental spectra With careful experiments and analysis, EELS can be a unique tool measuring the superconducting properties of MgB2, doped with various elements for improved transport properties on a sub-nanometer scale., Comment: 26 Pages, 5 figures, 1 table. Submited to PRB