Low-energy Electron Microscopy Research Articles

Thermal Instability of Gold Thin Films

The disintegration of a continuous metallic thin film leads to the formation of isolated islands, which can be used for the preparation of plasmonic structures. The transformation mechanism is driven by a thermally accelerated diffusion that leads to the minimalization of surface free energy in the system. In this paper, we report the results of our study on the disintegration of gold thin film and the formation of nanoislands on silicon substrates, both pure and with native silicon dioxide film. To study the processes leading to the formation of gold nanostructures and to investigate the effect of the oxide layer on silicon diffusion, metallic film with a thickness of 3 nm was deposited by molecular beam epitaxy (MBE) technique on both pure and oxidized silicon substrates. Transformation of the thin film was observed by low-energy electron microscopy (LEEM) and a scanning electron microscope (SEM), while the nanostructures formed were observed by atomic force microscope (AFM) method. Structural investigations were performed by low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) methods. Our experiments confirmed a strong correlation between the formation of nanoislands and the presence of native oxide on silicon substrates.

Dynamic topological domain walls driven by lithium intercalation in graphene.

Stacking engineering in van der Waals (vdW) materials is a powerful method to control topological electronic phases for quantum device applications. Atomic intercalation into the vdW material can modulate the stacking structure at the atomic scale without a highly technical protocol. Here we report that lithium intercalation in a topologically structured graphene/buffer system on SiC(0001) drives dynamic topological domain wall (TDW) motions associated with stacking order change by using an in situ aberration-corrected low-energy electron microscope in combination with theoretical modelling. We observe sequential and selective lithium intercalation that starts at topological crossing points (AA stacking) and then selectively extends to AB stacking domains. Lithium intercalation locally changes the domain stacking order to AA and in turn alters the neighbouring TDW stacking orders, and continuous intercalation drives the evolution of the whole topological structure network. Our work reveals moving TDWs protected by the topology of stacking and lays the foundation for controlling the stacking structure via atomic intercalation. These findings open up new avenues to realize intercalation-driven vdW electronic devices.

Study of Graphene by Scanning Low Energy Electron Microscopy and Time-of-Flight Spectroscopy.

Study of Graphene by Scanning Low Energy Electron Microscopy and Time-of-Flight Spectroscopy.

Steady-state junction current distribution in p-n GaN diodes measured using low-energy electron microscopy (LEEM)

We report on the measurement of the lateral distribution of the junction current of an electrical biased p-n GaN diode by electron emission microscopy using a low-energy electron microscope. The vacuum level at the surface of the diode was lowered by deposition of cesium to achieve negative electron affinity, allowing overflow electrons at the surface of the biased diodes to be emitted and their spatial distribution imaged. The results were compared to the literature, and a good match with analytical solutions by Joyce and Wemple [J. Appl. Phys. 41, 3818 (1970)] was obtained.

Super-resolution of magnetic systems using deep learning

We construct a deep neural network to enhance the resolution of spin structure images formed by spontaneous symmetry breaking in the magnetic systems. Through the deep neural network, an image is expanded to a super-resolution image and reduced to the original image size to be fitted with the input feed image. The network does not require ground truth images in the training process. Therefore, it can be applied when low-resolution images are provided as training datasets, while high-resolution images are not obtainable due to the intrinsic limitation of microscope techniques. To show the usefulness of the network, we train the network with two types of simulated magnetic structure images; one is from self-organized maze patterns made of chiral magnetic structures, and the other is from magnetic domains separated by walls that are topological defects of the system. The network successfully generates high-resolution images highly correlated with the exact solutions in both cases. To investigate the effectiveness and the differences between datasets, we study the network’s noise tolerance and compare the networks’ reliabilities. The network is applied with experimental data obtained by magneto-optical Kerr effect microscopy and spin-polarized low-energy electron microscopy.

Low energy electron microscopy at cryogenic temperatures

We describe a cryogenic sample chamber for low energy electron microscopy (LEEM), and present first experimental results. Modifications to our IBM/SPECS aberration-corrected LEEM instrument are presented first. These include incorporation of mechanisms for cooling the sample and its surroundings, and reduction of various sources of heat load. Using both liquid nitrogen and liquid helium, we have reached sample temperatures down to about 15 K. We also present first results for low-temperature LEEM, obtained on a three-monolayer pentacene film. Specifically, we observe a reduction of the electron beam irradiation damage cross-section at 15 eV by more than a factor of five upon cooling from 300 K down to 52 K. We also observe changes in the LEEM-IV spectra of the sample upon cooling, and discuss possible causes.

Gun energy filter for a low energy electron microscope

In a Low Energy Electron Microscope (LEEM) the sample is illuminated with an electron beam with typical electron landing energies from 0–100 eV. The energy spread of the electron beam is determined by the characteristics of the electron source. For the two most commonly used electron sources, LaB6 and cold field emission W, typical energy spreads ΔE are 0.75 and 0.25 eV at full width half maximum, respectively. Here we present a design for a LEEM gun energy filter, that reduces ΔE to ∼100 meV. Such a filter has been incorporated in the IBM/SPECS AC-LEEM system at IBM. Experimental results are presented and found to be in excellent agreement with expectations.

Visualization of molecular stacking using low-energy electron microscopy

The design of metal-organic interfaces with atomic precision enables the fabrication of highly efficient devices with tailored functionality. The possibility of fast and reliable analysis of molecular stacking order at the interface is of crucial importance, as the interfacial stacking order of molecules directly influences the quality and functionality of fabricated organic-based devices. Dark-field (DF) imaging using Low-Energy Electron Microscopy (LEEM) allows the visualization of areas with a specific structure or symmetry. However, distinguishing layers with different stacking orders featuring the same diffraction patterns becomes more complicated. Here we show that the top layer shift in organic molecular bilayers induces measurable differences in spot intensities of respective diffraction patterns that can be visualized in DF images. Scanning Tunneling Microscopy (STM) imaging of molecular bilayers allowed us to measure the shift directly and compare it with the diffraction data. We also provide a conceptual diffraction model based on the electron path differences, which qualitatively explains the observed phenomenon.

Preparation and stability of the hexagonal phase of samarium oxide on Ru(0001)

We have used low-energy electron microscopy (LEEM), micro-illumination low-energy electron diffraction (µLEED) supported by ab initio calculations, and X-ray absorption spectroscopy (XAS) to investigate in-situ and in real-time the structural properties of Sm2O3 deposits grown on Ru(0001), a rare-earth metal oxide model catalyst. Our results show that samarium oxide grows in a hexagonal A-Sm2O3 phase on Ru(0001), exhibiting a (0001) oriented-top facet and (113) side facets. Upon annealing, a structural transition from the hexagonal to cubic phase occurs, in which the Sm cations exhibit the +3 oxidation state. The unexpected initial growth in the A-Sm2O3 hexagonal phase and its gradual transition to a mixture with cubic C-Sm2O3 showcases the complexity of the system and the critical role of the substrate in the stabilization of the hexagonal phase, which was previously reported only at high pressures and temperatures for bulk samaria. Besides, these results highlight the potential interactions that Sm could have with other catalytic compounds with respect to the here gathered insights on the preparation conditions and the specific compounds with which it interacts.

Temporal coherence envelope function of field emission in electron microscopy

Imaging in electron microscopy is adversely affected by partial electron spatial and temporal coherence. Temporal coherence has been treated theoretically in the past using the method pioneered fifty years ago by Hanßen and Trepte, who assumed a Gaussian energy distribution. However, state-of-the-art instruments employ field emission (FE) sources that emit electrons with a non-Gaussian energy distribution. We have updated the treatment of temporal coherence to describe the effects of an arbitrary energy distribution on image formation. The updated approach is implemented in Fourier optics simulations to explore the effect of FE on image formation in conventional, non-aberration-corrected (NAC) and aberration-corrected (AC) low energy electron microscopy. It is found that the resolution that can be achieved for the FE distribution is only slightly degraded compared to a Gaussian distribution with the same energy spread. FE also produces a focus offset. These two effects are weaker for AC than for NAC microscopy. These and other insights may be relevant to the selection of the aperture size that optimizes resolution and to analyses that make use of focal image series. The approach developed here is also applicable to transmission electron microscopy.

Unraveling van der Waals epitaxy: A real-time in-situ study of MoSe2 growth on graphene/Ru(0001)

In the present work we investigate the growth of monolayer MoSe2 on selenium-intercalated graphene on Ru(0001), a model layered heterostructure combining a transition metal dichalcogenide with graphene, using low energy electron microscopy and micro-diffraction. Real-time observation of MoSe2 on graphene growth reveals the island nucleation dynamics at the nanoscale. Upon annealing, larger islands are formed by sliding and attachment of multiple nanometer-sized MoSe2 flakes. Local micro-spot angle-resolved photoemission spectroscopy reveals the electronic structure of the heterostructure, indicating that no charge transfer occurs within adjacent layers. The observed behavior is attributed to intercalation of Se at the graphene/Ru(0001) interface. The unperturbed nature of the proposed heterostructure therefore renders it as a model system for investigations of graphene supported TMD nanostructures.

60∘ and 120∘ domain walls in epitaxial BaTiO3 (111)/Co multiferroic heterostructures

We report on domain pattern transfer from a ferroelectric ${\mathrm{BaTiO}}_{3}$ substrate with a (111) orientation of the surface to an epitaxial Co film grown on a Pd buffer layer. Spatially modulated interfacial strain transfer from ferroelectric/ferroelastic domains and inverse magnetostriction in the ferromagnetic film induce stripe regions with a modulation of the in-plane uniaxial magnetic anisotropy direction. Using spin-polarized low-energy electron microscopy, we observe the formation of two distinct anisotropy configurations between stripe regions, leading to angles of ${60}^{\ensuremath{\circ}}$ or ${120}^{\ensuremath{\circ}}$ between the magnetizations of adjacent domains. Moreover, through application of a magnetic field parallel or perpendicular to these stripes, head-to-head or head-to-tail magnetization configurations are initialized. This results in four distinct magnetic domain-wall types associated with different energies and domain widths, which, in turn, affects whether domain pattern transfer can be achieved.

Development of in situ characterization of two-dimensional materials grown on insulator substrates with spectroscopic photoemission and low energy electron microscopy

Ultrathin two-dimensional (2D) materials offer great potential for next-generation integrated circuit and optoelectronic devices. Chemical vapor deposition (CVD)-grown 2D materials provide a way to mass production in industry. However, how to in situ characterize their intrinsic electric/photoelectric properties and carrier dynamics with electron/photoelectron probes is still a problem due to the interference from the conducting substrate. Here, we present a grounding Au grids method to realize in situ characterization of the CVD-grown MoS2 on the insulating thick SiO2 layer covered Si substrate with spectroscopic photoemission and low energy electron microscopy (SPELEEM). Through depositing Au grids afterwards, we have achieved good grounding of MoS2 flakes in the photoemission electron microscopy (PEEM), mirror electron microscopy (MEM), and micro-area low energy electron diffraction (µ-LEED) measurements. We have clarified the false signal caused by stray photoelectrons originated from the Au stripes, and as well as the space charge effects induced by intense photoemission. We have also confirmed that time-resolved PEEM results are not affected by the stray signal, and by adopting a small light spot, both static and time-resolved micro-area photoelectron spectroscopy (µ-PES) can be unaffected by space charge effects. Our results provide a reliable way to in situ investigate 2D materials grown on insulating substrates by probing photoelectrons or backscattered electrons.

Stacking domain morphology in epitaxial graphene on silicon carbide

Terrace-sized, single-orientation graphene can be grown on top of a carbon buffer layer on silicon carbide by thermal decomposition. Despite its homogeneous appearance, a surprisingly large variation in electron transport properties is observed. Here, we employ Aberration-Corrected Low-Energy Electron Microscopy (AC-LEEM) to study a possible cause of this variability. We characterize the morphology of stacking domains between the graphene and the buffer layer of high-quality samples. Similar to the case of twisted bilayer graphene, the lattice mismatch between the graphene layer and the buffer layer at the growth temperature causes a moir\'e pattern with domain boundaries between AB and BA stackings. We analyze this moir\'e pattern to characterize the relative strain and to count the number of edge dislocations. Furthermore, we show that epitaxial graphene on silicon carbide is close to a phase transition, causing intrinsic disorder in the form of co-existence of anisotropic stripe domains and isotropic trigonal domains. Using adaptive geometric phase analysis, we determine the precise relative strain variation caused by these domains. We observe that the step edges of the SiC substrate influence the orientation of the domains and we discuss which aspects of the growth process influence these effects by comparing samples from different sources.

Cleaning and tailoring the Pt[formula omitted]Sn(111) surface for surface experiments

The cleaning process of the bimetallic Pt3Sn(111) surface has been studied by means of low-energy electron microscopy (LEEM), microspot low-energy electron diffraction (μ-LEED), and X-ray photoemission electron microscopy (XPEEM). Different cleaning procedures, performed under ultra-high vacuum conditions (UHV), including sputtering with argon ions and repeated cycles of annealing up to 1500K were investigated. In this work, we show that a clean Pt3Sn(111) surface of high structural quality with a sharp and brilliant (2 × 2) bulk reconstruction in LEED as well as a perfectly smooth surface with terraces of micron size can be achieved by sputtering, annealing at very high temperatures, followed by a subsequent slow (0.09K/s) and careful cooling procedure. Additionally, we show the possibility of tailoring the Sn concentration in the topmost layers of Pt3Sn(111) as a function of annealing temperature and subsequent cooling rate. Structural changes of the surface are induced by Sn segregation combined with a surface order–disorder transition at 1340K. Moreover, two new surface reconstructions depending on the cooling rate are reported.

Competition between exchange and magnetostatic energies in domain pattern transfer from BaTiO3 (111) to a Ni thin film

We use spin-polarized low-energy electron microscopy to investigate domain pattern transfer in a multiferroic heterostructure consisting of a (111)-oriented ${\mathrm{BaTiO}}_{3}$ substrate and an epitaxial Ni film. After in situ thick-film deposition and annealing through the ferroelectric phase transition, interfacial strain transfer from ferroelastic domains in the substrate and inverse magnetostriction in the magnetic thick film introduce a uniaxial in-plane magnetic anisotropy that rotates by ${60}^{\ensuremath{\circ}}$ between alternating stripe regions. We show that two types of magnetic domain wall can be initialized in principle. Combining experimental results with micromagnetic simulations, we show that a competition between the exchange and magnetostatic energies in these domain walls has a strong influence on the magnetic domain configuration.

Low-energy electron microscopy contrast of stacking boundaries: Comparing twisted few-layer graphene and strained epitaxial graphene on silicon carbide

Stacking domain boundaries occur in Van der Waals heterostacks whenever there is a twist angle or lattice mismatch between subsequent layers. Not only can these domain boundaries host topological edge states, imaging them has been instrumental in determining local variations in twisted bilayer graphene. Here, we analyze the mechanisms causing stacking domain boundary contrast in bright field low-energy electron microscopy (BF-LEEM) both for graphene on SiC, where domain boundaries are caused by strain, and for twisted few-layer graphene. We show that when domain boundaries are between the top two graphene layers, BF-LEEM contrast is observed due to amplitude contrast and corresponds well to calculations of the contrast based purely on the local stacking in the domain boundary. Conversely, for deeper-lying domain boundaries, amplitude contrast only provides a weak distinction between the inequivalent stackings in the domains themselves. However, for small domains, electrons from different parts of the unit cell interfere, causing a very strong phase contrast. We derive a general rule-of-thumb of expected BF-LEEM contrast for domain boundaries in Van der Waals materials.

MAXPEEM: a spectromicroscopy beamline at MAX IV laboratory.

MAXPEEM, a dedicated photoemission electron microscopy beamline at MAX IV Laboratory, houses a state-of-the-art aberration-corrected spectroscopic photoemission and low-energy electron microscope (AC-SPELEEM). This powerful instrument offers a wide range of complementary techniques providing structural, chemical and magnetic sensitivities with a single-digit nanometre spatial resolution. The beamline can deliver a high photon flux of≥1015 photons s-1 (0.1% bandwidth)-1 in the range 30-1200 eV with full control of the polarization from an elliptically polarized undulator. The microscope has several features which make it unique from similar instruments. The X-rays from the synchrotron pass through the first beam separator and impinge the surface at normal incidence. The microscope is equipped with an energy analyzer and an aberration corrector which improves both the resolution and the transmission compared with standard microscopes. A new fiber-coupled CMOS camera features an improved modulation transfer function, dynamic range and signal-to-noise ratio compared with the traditional MCP-CCD detection system.

Photoluminescence of tin dioxide (SnO2) nanostructure grown on Si(001) by thermal evaporation technique

This work reports the growth and photoluminescence of tin dioxide (SnO2) nanostructure on Si (001) substrate. The growth was done by thermal evaporation of tin and subsequent air exposure at room temperature. The in situ low energy electron microscopy and atomic force microscopy studies indicated that the growth followed the Volmer-Werber growth mode leading to the formation of SnO2 island nanostructure with the smallest island size of 86 ± 10 nm and the highest percentage of surface coverage of 53% at the substrate temperature of 225 °C. The grazing incident X-ray diffraction and X-ray photoelectron spectroscopy results confirmed the formation of amorphous SnO2 nanostructure. The photoluminescence spectra under the excitation of 350 nm radiation at room temperature showed two emission peaks in the yellow region at 574.0 nm and 555.9 nm, and three emission peaks in the blue region at 450 nm, 467 nm, and 478 nm. The CIE (x,y) chromaticity calculated from the emission spectra showed that the coordinators were located at approximately x = 0.2419 and y = 0.3089 in a region of white emission.

Early-stage growth of GeTe on Si(111)-Sb

The advent of germanium telluride as a promising ferroelectric Rashba semiconductor for spintronic applications requires the growth of nanometer-thick films of high crystalline quality. In this study, we have elucidated the initial growth stages of GeTe on Si(111)-Sb by scanning tunneling microscopy and low energy electron diffraction. We demonstrate the presence of an initial 0.35-nm-thick GeTe buffer layer followed by the 2D growth of GeTe via Frank-Read sources of atomic steps. As shown by core level spectroscopy, Sb is acting as a surfactant during growth up to a 5-nm-thick film. X-ray diffraction, transmission electron microscopy, and low energy electron microscopy evidence that numerous mirror domains and in-plane misorientations appear early in the growth process and are gradually buried at the film/substrate interface. The use of a miscut Si substrate close to Si(111) allows suppressing these defects from the early beginning of growth.