High Resolution Electron Energy Loss Research Articles

Phonon modes and electron-phonon coupling at the FeSe/SrTiO3 interface.

The remarkable increase in superconducting transition temperature (Tc) observed at the interface of one-unit-cell FeSe films on SrTiO3 substrates (1 uc FeSe/STO)1 has attracted considerable research into the interface effects2-6. Although this high Tc is thought to be associated with electron-phonon coupling (EPC)2, the microscopic coupling mechanism and its role in the superconductivity remain elusive. Here we use momentum-selective high-resolution electron energy loss spectroscopy to atomically resolve the phonons at the FeSe/STO interface. We uncover new optical phonon modes, coupling strongly with electrons, in the energy range of 75-99 meV. These modes are characterized by out-of-plane vibrations of oxygen atoms in the interfacial double-TiOx layer and the apical oxygens in STO. Our results also demonstrate that the EPC strength and superconducting gap of 1 uc FeSe/STO are closely related to the interlayer spacing between FeSe and the TiOx terminated STO. These findings shed light on the microscopic origin of the interfacial EPC and provide insights into achieving large and consistent Tc enhancement in FeSe/STO and potentially other superconducting systems.

Advanced spectroscopic methods for probing in-gap defect states in amorphous SiNx for charge trap memory applications

Silicon nitride (SiNx) serves as the charge trap layer in current 3D NAND flash memory devices. The precise formation mechanism and electronic structure of localized defect trap states in SiNx remain elusive. Here, we present a refined experimental methodology to elucidate the in-gap defect states and the band gaps in amorphous SiNx thin films. Our approach integrates high-resolution reflection electron energy loss spectroscopy (REELS) and spectroscopic ellipsometry (SE) for comprehensive analysis. By systematical analysis, we aim to provide a robust method for determining in-gap electronic states in SiNx. We investigated two different SiNx films prepared by plasma-enhanced chemical vapor deposition and sputtering. Our analysis revealed several distinct in-gap states and determined band gap energies. This approach not only provide advanced spectroscopic methods to characterize the defect electronic states in SiNx, but also applicable to other large band gap semiconductors or dielectrics to predict device-level characteristics for future devices.

Embedded Hybrid-Dimensional Heterointerface for Filament Modulation in 2D Material-Based Artificial Nociceptor.

Nociceptors are key sensory receptors that transmit warning signals to the central nervous system in response to painful stimuli. This fundamental process is emulated in an electronic device by developing a novel artificial nociceptor with an ultrathin, nonstoichiometric gallium oxide (GaOx)-silicon oxide heterostructure. A large-area 2D-GaOx film is printed on a substrate through liquid metal printing to facilitate the production of conductive filaments. This nociceptive structure exhibits a unique short-term temporal response following stimulation, enabling a facile demonstration of threshold-switching physics. The developed heterointerface 2D-GaOx film enables the fabrication of fast-switching, low-energy, and compliance-free 2D-GaOx nociceptors, as confirmed through experiments. The accumulation and extrusion of Ag in the oxide matrix are significant for inducing plastic changes in artificial biological sensors. High-resolution transmission electron microscopy and electron energy loss spectroscopy demonstrate that Ag clusters in the material dispersed under electrical bias and regrouped spontaneously when the bias is removed owing to interfacial energy minimization. Moreover, 2D nociceptors are stable; thus, heterointerface engineering can enable effective control of charge transfer in 2D heterostructural devices. Furthermore, the diffusive 2D-GaOx device and its Ag dynamics enable the direct emulation of biological nociceptors, marking an advancement in the hardware implementation of artificial human sensory systems.

Fabrication of monolayer h-BN/LaB6 heterostructure thin film with low work function and high chemical stability

To expand the applications of low work function materials, high chemical stability is necessary to prevent the surface from unfavorable reactions. We developed a 20-nm-thick lanthanum hexaboride (LaB6) thin film covered by a monolayer hexagonal boron nitride (h-BN), which exhibits not only low work function but also high chemical stability. Results demonstrated that the h-BN/LaB6 heterostructure can be formed by vacuum annealing a nitrogen-doped LaB6 thin film. The formation process was investigated using Auger electron spectroscopy (AES), high-resolution electron energy loss spectroscopy (HREELS), and X-ray absorption near edge structure (XANES) spectroscopy. We have elucidated that the doped nitrogen atoms and boron atoms in the film thermally diffuse into the surface during annealing, thereby forming the monolayer h-BN on the surface. Work function was measured using scanning tunneling microscopy (STM). From a practical perspective, chemical stability was evaluated with a heating temperature necessary for restoring the low work function state after air exposure. The work function was comparable to that of the clean LaB6(100) surface. Moreover, it recovered at a much lower temperature than the cleaning temperature of the LaB6(100) surface. We anticipate that this material development will facilitate implementation of the low work function material.

The Influence of Carbon on Polytype and Growth Stability of Epitaxial Hexagonal Boron Nitride Films

AbstractBoron nitride (BN) is a promising 2D material as well as a potential wide‐bandgap semiconductor. Chemical vapor deposition (CVD) is commonly used to deposit single layers or thin films of BN, but the deposition process is insufficiently understood at an atomic scale. the CVD of BN is studied using two boron precursors, the organoboranes, triethylborane, and trimethylborane. Using high resolution (scanning) transmission electron microscopy and electron energy loss spectroscopy, this study shows that hexagonal‐BN (h‐BN) nucleates and grows epitaxially for ≈4 nm before it either polytype transforms to rhombohedral‐BN (r‐BN), turns to less ordered turbostratic‐BN or is terminated by a layer of amorphous carbon. this study proposes that the carbon in the organoboranes deposits on the epitaxially growing h‐BN surface and this either leads to the polytype transition to r‐BN, the transition to less ordered BN growth, or complete surface poisoning with carbon terminating BN growth. These results question the use of organoboranes in CVD of epitaxial BN films, and the polytype stability of h‐BN growing on graphene.

4H-SiC/SiO2 Interface Degradation in 1.2 kV 4H-SiC MOSFETs due to Power Cycling Tests

Power cycling tests (PCTs) assess the reliability of power devices by closely simulating their operating conditions. A PCT was performed on commercially available 1.2 kV 4H-SiC power metal–oxide–semiconductor field-effect transistors to observe its impact on the 4H-SiC/SiO2 interface. High-resolution transmission electron microscopy and electron energy loss spectroscopy measurements showed variations in the length of the 4H-SiC/SiO2 transition layer, depending on whether the device was power cycled. Moreover, the total resistance at Vg >> Vt in Rtot − (Vg-Vt)−1 graph increased to 16.5%, while it changed more radically to 47.3% at Vg ≈ Vt. The threshold voltage shifted negatively. These variations cannot be expected solely through the wearout of the package.

Oxygen Transport through Amorphous Cathode Coatings in Solid-State Batteries.

All solid-state batteries (SSBs) are considered the most promising path to enabling higher energy-density portable energy, while concurrently improving safety as compared to current liquid electrolyte solutions. However, the desire for high energy necessitates the choice of high-voltage cathodes, such as nickel-rich layered oxides, where degradation phenomena related to oxygen loss and structural densification at the cathode surface are known to significantly compromise the cycle and thermal stability. In this work, we show, for the first time, that even in an SSB, and when protected by an intact amorphous coating, the LiNi0.5Mn0.3Co0.2O2 (NMC532) surface transforms from a layered structure into a rocksalt-like structure after electrochemical cycling. The transformation of the surface structure of the Li3B11O18 (LBO)-coated NMC532 cathode in a thiophosphate-based solid-state cell is characterized by high-resolution complementary electron microscopy techniques and electron energy loss spectroscopy. Ab initio molecular dynamics corroborate facile transport of O2- in the LBO coating and in other typical coating materials. This work identifies that oxygen loss remains a formidable challenge and barrier to long-cycle life high-energy storage, even in SSBs with durable, amorphous cathode coatings, and directs attention to considering oxygen permeability as an important new design criteria for coating materials.

Probing post-growth hydrogen intercalation and H2 nanobubbles formation in graphene on Ge(110)

We investigate the reproducibility of repeated intercalation of hydrogen in graphene/Ge (110) and the formation of H2 nanobubbles after thermal treatments. By exploiting high-resolution electron energy loss, we obtain direct spectroscopic fingerprints of H2 trapped gas in the samples when nanobubbles are present and we are able to track the effectiveness of H intercalation via the Ge–H vibrational mode. We correlate the effectiveness of interface re-hydrogenation to the presence of structural defects in graphene as highlighted by Raman spectroscopy. The π-plasmon mode of graphene on Ge (110) is investigated as a function of the hydrogen presence at the interface, revealing that, independent of the hydrogen intercalation status, graphene is weakly interacting on Ge (110).

Momentum resolved band gap measurement by high energy resolution electron energy loss spectroscopy

Momentum resolved band gap measurement by high energy resolution electron energy loss spectroscopy

On the origin of epitaxial rhombohedral-B4C growth by CVD on 4H-SiC.

Rhombohedral boron carbide, often referred to as r-B4C, is a potential material for applications in optoelectronic and thermoelectric devices. From fundamental thin film growth and characterization, we investigate the film-substrate interface between the r-B4C films grown on 4H-SiC (0001̄) (C-face) and 4H-SiC (0001) (Si-face) during chemical vapor deposition (CVD) to find the origin for epitaxial growth solely observed on the C-face. We used high-resolution (scanning) transmission electron microscopy and electron energy loss spectroscopy to show that there is no surface roughness or additional carbon-based interlayer formation for either substrate. Based on Raman spectroscopy analysis, we also argue that carbon accumulation on the surface hinders the growth of continued epitaxial r-B4C in CVD.

Nanocarbon oxidation in the environmental transmission electron microscope - Disentangling the role of the electron beam

Environmental transmission electron microscopy (ETEM) can provide unique insights into nanocarbon oxidation processes through atomic resolution and real time imaging of materials at high temperatures in reactive atmospheres. However, the electron beam can also influence the reaction rates, and even alter the processes entirely, complicating the interpretation of the in situ observations. Many mechanisms have been proposed to account for the impact of the electron beam, predominantly involving ionization of the oxidative gases to form more reactive species. However, these mechanisms have not been critically evaluated and compared to predictions from theory. Here, we evaluate the impact of the electron beam both qualitatively (oxidation mode and spatial extent) and quantitatively (oxidation rates), using high resolution imaging and electron energy loss spectroscopy, at different electron energies and dose rates. We demonstrate that transient defects generated by elastic scattering, forming highly active sites for carbon abstraction by oxygen, is the main mechanism for the enhanced oxidation rates observed in situ. This is evident from an insensitivity to electron energy and saturation of the effects at high electron dose rates. To avoid undue influence of the electron beam in future ETEM studies, we therefore recommend conditions where the intrinsic oxidation dominates over the beam-enhanced oxidation (note that no conditions are completely “safe”) and extensive comparisons with other methods.

Metal Core-Shell Nanoparticle Supercrystals: From Photoactivation of Hydrogen Evolution to Photocorrosion.

Gas nanobubbles are directly linked to many important chemical reactions. While they can be detrimental to operational devices, they also reflect the local activity at the nanoscale. Here, supercrystals made of highly monodisperse Ag@Pt core-shell nanoparticles are first grown onto a solid support and fully characterized by electron microscopies and X-ray scattering. Supercrystals are then used as a plasmonic photocatalytic platform for triggering the hydrogen evolution reaction. The catalytic activity is measured operando at the single supercrystal level by high-resolution optical microscopy, which allows gas nanobubble nucleation to be probed at the early stage with high temporal resolution and the amount of gas molecules trapped inside them to be quantified. Finally, a correlative microscopy approach and high-resolution electron energy loss spectroscopy help to decipher the mechanisms at the origin of the local degradation of the supercrystals during catalysis, namely nanoscale erosion and corrosion.

Direct Observation of Topological Phonons in Graphene.

Phonons, as the most fundamental emergent bosons in condensed matter systems, play an essential role in the thermal, mechanical, and electronic properties of crystalline materials. Recently, the concept of topology has been introduced to phonon systems, and the nontrivial topological states also exist in phonons due to the constraint by the crystal symmetry of the space group. Although the classification of various topological phonons has been enriched theoretically, experimental studies were limited to several three-dimensional (3D) single crystals with inelastic x-ray or neutron scatterings. The experimental evidence of topological phonons in two-dimensional (2D) materials is absent. Here, using high-resolution electron energy loss spectroscopy following our theoretical predictions, we directly map out the phonon spectra of the atomically thin graphene in the entire 2D Brillouin zone, and observe two nodal-ring phonons and four Dirac phonons. The closed loops of nodal-ring phonons and the conical structure of Dirac phonons in 2D momentum space are clearly revealed by our measurements, in nice agreement with our theoretical calculations. The ability of 3D mapping (2D momentum space and energy space) of phonon spectra opens up a new avenue to the systematic identification of the topological phononic states. Our work lays a solid foundation for potential applications of topological phonons in superconductivity, dynamic instability, and phonon diode.

Automatic and Quantitative Measurement of Spectrometer Aberrations.

The performance of electron energy loss spectrometers can often be limited by their electron optical aberrations. Due to recent developments in high energy resolution and momentum-resolved electron energy loss spectroscopy (EELS), there is renewed interest in optimizing the performance of such spectrometers. For example, the "ω - q" mode of momentum-resolved EELS, which uses a small convergence angle and requires aligning diffraction spots with the slot aperture, presents a challenge in the realignments of the spectrometer required by the adjustment of the projection lenses. Automated and robust alignment can greatly benefit such a process. The first step toward this goal is automatic and quantitative measurement of spectrometer aberrations. We demonstrate the measurement of geometric aberrations and distortions in EELS within a monochromated scanning transmission electron microscope (STEM). To better understand the results, we present a wave mechanical simulation of the experiment. Using the measured aberration and distortion coefficients as inputs to the simulation, we find a good match between the simulation and experiment, verifying formulae used in the simulation. From verified simulations with known aberration coefficients, we can assess the accuracy of measurements. Understanding the errors and inaccuracies in the procedure can guide further progress in aberration measurement and correction for new spectrometer developments.

Characterization of Cu-Cu direct bonding in ambient atmosphere enabled using (111)-oriented nanotwinned-copper

The copper-copper (Cu-Cu) direct bonding technology was proposed in recent years in order to realize precise preparation and reliable service under high current density for three-dimension (3D) packaging in the post-Moore-era. In this study, Cu-Cu direct bonding was achieved in the ambient atmosphere by utilizing highly (111) oriented copper with nanotwin structure (called (111) nt-Cu). The thermal compression with a pressure of 30 MPa was conducted on a couple of Cu films at 300 and 400 °C in air. The Cu-Cu joints after bonding were further investigated by scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). Different from the polycrystalline copper, perfect bonding was achieved as confirmed by high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). The shear strength after air bonding was measured as 96.04 ± 5.67 MPa on average, which demonstrated relatively high performance in traditional Cu-Cu bonding. The excellent resistance to oxidation and fast Cu(111) surface diffusivity of (111) nt-Cu guaranteed the bonding process. Above all, the achievement of (111) nt-Cu high performance bonding in the ambient atmosphere should give an insight into developing new interconnected technology.

Mechanistic Understanding of Ring-Opening of Tetrahydrofurfuryl Alcohol over WOx-Modified Pt Model Surfaces and Powder Catalysts

The upgrading of heterocyclic biomass-derived oxygenates such as tetrahydrofurfuryl alcohol (THFA) via ring-opening is a promising pathway to produce value-added diol molecules using renewable carbon sources. This study combines model surface experiments, first-principles calculations, and powder catalyst characterization and activity evaluation to unravel the nature of the Pt and WOx active sites and the reaction mechanism of the THFA ring-opening reaction on a WOx/Pt inverse oxide catalyst. Temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) measurements on model surfaces demonstrated that THFA ring opened on Pt(111) but underwent further decomposition due to its strong bonding with the surface. However, WOx deposited on Pt(111) altered the interaction strength between the ring-opened intermediate and the surface to a proper extent to facilitate the facile desorption of the desired 1,5-pentanediol (1,5-PeD) product. Density functional theory (DFT) calculations showed that WOx/Pt(111) could promote ring opening of THFA via an oxocarbenium ion-like transition state, which was stabilized by hydrogen bonding with the hydroxyl groups of WOx. The hydrogenation of the ring-opened 5-hydroxyvaleraldehyde intermediate to 1,5-PeD was then feasible via Brønsted acid sites present on WOx. Steady-state activity studies on the corresponding powder catalysts showed that the 1,5-PeD selectivity increased from 20% on Pt/SiO2 to 65% on WOx/Pt/SiO2 with 1 wt % WOx loading, consistent with model surface experiments and DFT calculations. This study demonstrates the feasibility of using model surface experiments and first-principles calculations to guide practical catalyst design, and provides a design strategy that can be applied to the selective ring-opening of relevant heterocyclic biomass-derived oxygenates.

How positioning of a hard ceramic TiB2 layer in Al/CuO multilayers can regulate the overall energy release behavior

This study reports on a new ternary thermite comprising of Al-TiB2/CuO multilayers, designed to take the advantage of high ignitability of titanium (Ti), high volumetric density of boron (B), and low melting point of aluminum (Al). Results demonstrate synergetic effects leading to an energetic layer outperforming its single fuel counterparts, Al/CuO or TiB2/CuO, while being safe to handle. The additive TiB2 not only lowers the ignition energy by 100%, but also enhances the burn rate by more than a factor of two compared to the single fuel samples (Al/CuO and TiB2/CuO). The thermite reaction sequences and synergy of Ti, B and Al oxidation, examined by thermo-analytical analyses coupled with X-ray spectroscopy and high-resolution electron energy loss spectroscopy, demonstrate the strong affinity of TiB2 to oxygen that catalyzes CuO decomposition at temperatures as low as 380 °C; followed by a dual-step TiB2 oxidation: first to TiO, and then to TiO2 led by a reaction limiting step of oxidizer availability. Finally, Al undergoes oxidation via liquid boron oxide as well as gaseous oxygen. This study also underlines the crucial effect of the nanolayer morphology in the thin-film technology: when Al is sputter-deposited onto the TiB2 layer, Al ions penetrate into the grain boundaries penalizing TiB2 reactivity. The results demonstrate the advantages of using TiB2 fuel to improve the ignitability and the combustion performance of Al based thermite and offer some means to finely tune the energetic properties.

Nodal-line plasmons in ZrSiX ( X=S,Se,Te ) and their temperature dependence

Topological nodal-line semimetals gain extensive attention for their peculiar linear band crossings along closed lines in the Brillouin zone leading to various interesting properties. Here, we systematically study the nodal-line plasmons of prototypical nodal-line semimetals $\mathrm{ZrSi}X$ ($X=\mathrm{S},\mathrm{Se},\mathrm{Te}$) by high-resolution electron energy loss spectroscopy, and first-principles calculations. In energy range from near- to mid-infrared frequencies, plasmons induced by the intraband correlations ($\ensuremath{\sim}0.8$ eV) and the interband correlations ($\ensuremath{\sim}0.3$ eV) of surface states related to the nodal-line electrons are universally observed in all the $\mathrm{ZrSi}X$ family. The bulk plasmon ($\ensuremath{\sim}0.6$ eV) is observed in ZrSiS but is indiscernible in both ZrSiSe and ZrSiTe owing to their stronger screening effect from the surface states and large interlayer distance. Although the plasmons in ZrSiS seem to be temperature independent, the frequencies of the intraband plasmons in ZrSiSe and ZrSiTe show large redshifts ($\ensuremath{\sim}10%$) with temperature increasing from 30 K to room temperature, which can be ascribed to the prominent thermal occupation effect of nodal-line electrons when the Fermi level is close to the nodal line.

The effect of ammonia addition on soot nanostructure and composition in ethylene laminar flames

The formation of soot in ammonia (NH3) combustion with a hydrocarbon such as ethylene (C2H4) is investigated using the analysis of particles’ nanostructure and surface chemical composition to identify the mechanisms by which NH3 suppresses soot growth. Young and mature soot particles were extracted from laminar diffusion co-flow flames of NH3C2H4 blends up to 50% NH3. The soot nanostructure is examined using lattice fringe analysis of high-resolution transmission electron microscopy (HRTEM) images, Raman Spectroscopy, and Electron Energy Loss Spectroscopy (EELS). The chemical composition of the soot surface is analyzed using X-ray Photoelectron Spectroscopy (XPS). With increasing NH3 addition, reduced planar growth of the soot graphitic layers is observed through the analysis of the soot nanostructure which indicates the suppression of carbon addition at the edge sites of the layer planes. It is also found that the carbon sp2/sp3 bonding ratio increases which shows a nanostructure with less carbon sp3 bonding at defect sites. By exploring the chemical composition, the nitrogen content of the soot surface increases with NH3 addition which is caused by the bonding of nitrogenated species with carbon at defect and edge sites of the carbon layer, reducing the potential for carbon addition, and thus soot growth. The results also show an increased risk of formation of nitrogenated polyaromatic hydrocarbons (N-PAHs) on the soot surface by NH3 addition. The study presents a novel investigation of the impact of NH3 on the soot surface chemistry and particles nanostructure which explains the role of NH3 in suppressing soot growth and reveals the potential for new toxic characteristics for the soot emissions formed in NH3-hydrocarbon cofired systems.

Nanoscale chemical and structural investigation of solid solution polyelemental transition metal oxide nanoparticles

Although it has been shown that configurational entropy can improve the structural stability in transition metal oxides (TMOs), little is known about the oxidation state of transition metals under random mixing of alloys. Such information is essential in understanding the chemical reactivity and properties of TMOs stabilized by configurational entropy. Herein, utilizing electron energy loss spectroscopy (EELS) technique in an aberration-corrected scanning transmission electron microscope (STEM), we systematically studied the oxidation state of binary (Mn,Fe)3O4, ternary (Mn, Fe, Ni)3O4, and quinary (Mn, Fe, Ni, Cu, Zn)3O4 solid solution polyelemental transition metal oxides (SSP-TMOs) nanoparticles. Our findings show that the random mixing of multiple elements in the form of solid solution phase not only promotes the entropy stabilization but also results in stable oxidation state in transition metals spanning from binary to quinary transition metal oxide nanoparticles.