Photoemission Electron Microscopy Images Research Articles

Photoemission Electron Microscopy of Blue and UV Surface Plasmons on Nanostructured Aluminum Films

We image surface plasmons on aluminum surfaces in the ultraviolet and blue spectral range with photoemission electron microscopy (PEEM). Plasmon excitation is mediated by light incident on straight aluminum ridges with subwavelength cross section on flat aluminum films. We observe interference patterns due to light/plasmon interaction, with distinct periodicity and orientation depending on the light in-plane incidence angle with respect to the ridge orientation. From the interference patterns, we deduce the surface plasmon propagation directions and decay lengths. The latter are remarkably high with 2.8 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu$$\\end{document}m and 8.4 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu$$\\end{document}m for excitation wavelengths of 273 nm and 410 nm, respectively. Our results extend PEEM imaging of surface plasmons into the ultraviolet region.

Imaging Phase Segregation in Nanoscale LixCoO2 Single Particles

LixCoO2 (LCO) is a common battery cathode material that has recently emerged as a promising material for other applications including electrocatalysis and as electrochemical random access memory (ECRAM). During charge-discharge cycling LCO exhibits phase transformations that are significantly complicated by electron correlation. While the bulk phase diagram for an ensemble of battery particles has been studied extensively, it remains unclear how these phases scale to nanometer dimensions and the effects of strain and diffusional anisotropy at the single-particle scale. Understanding these effects is critical to modeling battery performance and for predicting the scalability and performance of electrocatalysts and ECRAM. Here we investigate isolated, epitaxial LiCoO2 islands grown by pulsed laser deposition. After electrochemical cycling of the islands, conductive atomic force microscopy (c-AFM) is used to image the spatial distribution of conductive and insulating phases. Above 20 nm island thicknesses, we observe a kinetically arrested state in which the phase boundary is perpendicular to the Li-planes; we propose a model and present image analysis results that show smaller LCO islands have a higher conductive fraction than larger area islands, and the overall conductive fraction is consistent with the lithiation state. Thinner islands (14 nm), with a larger surface to volume ratio, are found to exhibit a striping pattern, which suggests surface energy can dominate below a critical dimension. When increasing force is applied through the AFM tip to strain the LCO islands, significant shifts in current flow are observed, and underlying mechanisms for this behavior are discussed. The c-AFM images are compared with photoemission electron microscopy images, which are used to acquire statistics across hundreds of particles. The results indicate that strain and morphology become more critical to electrochemical performance as particles approach nanometer dimensions.

Improvement of femtosecond SPPs imaging by two-color laser photoemission electron microscopy

Clear imaging of surface plasmon polaritons (SPPs) is a prerequisite for SPPs-based applications. In this work, we demonstrate an improvement of near-field imaging of SPPs via directly comparing the visibility of the photoemission electron microscopy (PEEM) image of SPPs under one- and two-color laser excitation (also known as one- or two-color laser PEEM). By measuring the photoelectron yield and the contrast of the interference fringes of SPPs, we demonstrate that in addition to enhancing the photoemission yield, two-color laser PEEM can significantly improve the contrast between bright and dark fringes (nearly 4 times higher than that of one-color laser case). By recording the nonlinear order of the photoelectrons ejected from the bright and dark fringes, respectively, the underlying mechanism for the improved visibility is revealed. In addition, the influences of the polarization direction of 400-nm laser on the PEEM images of the SPPs with different wave vector directions are shown. These results can provide technical support for the development of SPPs-based communication devices and catalysis.

Surface functionalization of few-layer graphene on [formula omitted]-SiC(001) by Neutral Red dye

Few-layer graphene on β-SiC(001) functionalized with phenazine dye Neutral Red by means of diazonium chemistry has been studied using X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure, photoemission electron microscopy, scanning tunneling microscopy, and density functional theory calculations. The experimental data reveal the formation of a composite phenazine dye/graphene structure with a large energy gap. The molecules in this structure can be oriented both parallel and perpendicular to the graphene surface. According to scanning tunneling spectroscopy and theoretical calculations, the density of electron states in different surface areas depends on the local short-range order and the molecules’ environment. On the other hand, the photoemission spectroscopy study shows that the bottom layers of the few-layer graphene remain intact, which inherently makes the synthesized layered composite a low-dimensional metal/semiconductor heterostructure. In addition, photoemission electron microscopy imaging shows a high homogeneity of the dye-modified graphene on a micrometer scale.

Investigation of a dual-hole structure-based broadband femtosecond nondiffracting SPP beam emitter by photoemission electron microscopy

Nondiffracting surface plasmon polariton (SPP) beam, which has unique self-healing and non-divergence properties, can be a good candidate in the next-generation on-chip devices and has attracted considerable attention. Here, a simple and broadband coupled femtosecond nondiffracting SPP beam emitter based on a dual-hole structure has been demonstrated and characterized by photoemission electron microscopy (PEEM). The PEEM experiments show that, compared with the usual one-color PEEM, the two-color PEEM scheme offers a clear image of the nondiffracting SPP by effectively reducing the influence of the light-SPP interference field. As a result, the parameters of the nondiffracting SPP beam, such as the divergence angle, waist width, and propagation distance, are experimentally determined. The PEEM images of the nondiffracting SPP are well reproduced by the finite-difference time-domain (FDTD) method. This work plays a vital role in the further study of, e.g., high spatial and temporal characterization of the dynamic evolution of nondiffracting SPP beam.

Plasmonic spin-Hall effect of propagating surface plasmon polaritons in Ni80Fe20 microstructures

Photoexcitation and shaping of a propagating surface plasmon polariton (SPP) on silver and gold microstructures are well established and lead to the discovery of the plasmonic spin-Hall effect recently. Whereas silver is often the material of choice due to its exceptional low plasma frequency and weak damping, similar observations have not been reported for ferromagnetic metals. In this work, we report on propagating SPPs on Ni80Fe20 microstructures imaged by photoemission electron microscopy (PEEM) in combination with a tunable femtosecond laser system at MHz repetition rate. Circular dichroic (CD) images in threshold PEEM show clear edge-induced SPPs with sub-micrometer wavelength and propagation length of about 3.5 μm. Analysis of the interference patterns as well as the coupling of the optical spin angular momentum to the observed fringe fields reveal propagation characteristics exclusive to evanescent waves and the presence of the plasmonic spin-Hall effect. Our work provides direct evidence that many materials with a high plasma frequency allow for excitation and observation of propagating SPPs at the dielectric/metal interface via CD PEEM imaging, enabling magnetoplasmonic investigation of common ferromagnets on nanometer length and femtosecond time scales.

Symmetrical double-groove structure as a broadband plasmonic polarization multiplexer

Plasmonic polarization multiplexers are becoming increasingly significant for the development of high-speed plasmonic interconnection with broad bandwidth. Here, we demonstrate that the symmetrical double-groove structure can act as a polarization multiplexer for broadband femtosecond surface plasmon polaritons (SPPs) by investigating the mode distributions with photoemission electron microscopy (PEEM). Experimental results show that the mode distribution of the SPPs field from the excitation of the double-groove structure is symmetrical when irradiated with p-polarized light, whereas the mode shows an anti-symmetric field distribution as a staggered, herringbone shape with s-polarized light excitation. Furthermore, the SPPs field distribution shows an asymmetrical mode when the double-groove is excited with a left or right-handed circular femtosecond laser pulse. The resulting PEEM images are supported by FDTD simulations. The proposed multiplexer is a promising building block for highly integrated optoelectronic integrated circuits.

How the anisotropy of surface oxide formation influences the transient activity of a surface reaction

Scanning photoelectron microscopy (SPEM) and photoemission electron microscopy (PEEM) allow local surface analysis and visualising ongoing reactions on a µm-scale. These two spatio-temporal imaging methods are applied to polycrystalline Rh, representing a library of well-defined high-Miller-index surface structures. The combination of these techniques enables revealing the anisotropy of surface oxidation, as well as its effect on catalytic hydrogen oxidation. In the present work we observe, using locally-resolved SPEM, structure-sensitive surface oxide formation, which is summarised in an oxidation map and quantitatively explained by the novel step density (SDP) and step edge (SEP) parameters. In situ PEEM imaging of ongoing H2 oxidation allows a direct comparison of the local reactivity of metallic and oxidised Rh surfaces for the very same different stepped surface structures, demonstrating the effect of Rh surface oxides. Employing the velocity of propagating reaction fronts as indicator of surface reactivity, we observe a high transient activity of Rh surface oxide in H2 oxidation. The corresponding velocity map reveals the structure-dependence of such activity, representing a direct imaging of a structure-activity relation for plenty of well-defined surface structures within one sample.

Controlling Electric Field and Photoemission at the Tips of Triangular Gold Antennas

We show that the fields of surface plasmon polaritons (SPPs)—excited by infrared ultrafast laser light—can be highly concentrated at the corners of thin, micron-scale gold triangular platelets. We present multi-photon photoemission electron microscopy (nP-PEEM) images and finite element method (FEM) simulations that show that the magnitude of the SPP electric field can be adjusted over a large range via control of the polarization, wavelength, and angle of incidence of the applied laser light. As it is well known that the strength of SPP near-fields has a strong bearing on non-linear photoelectron emission rates, localized SPP fields might effectively be used as a nano-scale adjustable electron source.

Chiral asymmetry detected in a 2D array of permalloy square nanomagnets using circularly polarized x-ray resonant magnetic scattering

The sensitivity of circularly polarized x-ray resonant magnetic scattering (CXRMS) to chiral asymmetry has been demonstrated. The study was performed on a 2D array of Permalloy (Py) square nanomagnets of 700 nm lateral size arranged in a chess pattern, in a square lattice of 1000 nm lattice parameter. Previous x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) images on this sample showed the formation of vortices at remanence and a preference in their chiral state. The magnetic hysteresis loops of the array along the diagonal axis of the squares indicate a non-negligible and anisotropic interaction between vortices. The intensity of the magnetic scattering using circularly polarized light along one of the diagonal axes of the square magnets becomes asymmetric in intensity in the direction transversal to the incident plane at fields where the vortex states are formed. The asymmetry sign is inverted when the direction of the applied magnetic field is inverted. The result is the expected in the presence of an unbalanced chiral distribution. The effect is observed by CXRMS due to the interference between the charge scattering and the magnetic scattering.

Demonstrating a two-dimensional-tunable surface plasmon polariton dispersion element using photoemission electron microscopy

Functional elements of propagating surface plasmons are becoming increasingly significant for the development of plasmonic devices. To the best of our knowledge, the dispersion element of two-dimensional (2D) femtosecond surface plasmon polaritons (SPPs) as its central importance for future nano-optical circuits has never been reported. Here, we first demonstrate that the trench structure can act as a 2D-tunable SPP dispersion element by imaging the noncollinear mode using photoemission electron microscopy (PEEM). Experimental results show that directed propagation of SPPs can be controlled by varying the polarization direction of the femtosecond laser, and, more importantly, it is found that the actual propagation direction of the dispersed SPPs is directly obtained by reading PEEM images of SPPs using the noncollinear excited method in an Ag film. The experimental results are in agreement with classical 2D wave simulations. The results have demonstrated that the trench structure has potential as a tunable plasmonic dispersion element.

Pulse picking in synchrotron-based XPEEM

We report on a simple and cost-effective device for high-speed gating in photoemission electron microscopy (PEEM) with pulsed photon sources. This device is based on miniaturized electrode plates, which deflect the photoelectron beam inside the imaging column of the microscope so that it is either accepted or blocked in its path towards the detector. The gating device is optimized for installation on the Elmitec SPELEEM III microscope. Due to the compact design, it can be driven by voltage pulses of low amplitude (few volts), delivered by commercially available signal generators. Most notably, our device allows for stroboscopic data collection with on-time of less than 10 ns and at a rate in the range from 1 MHz to 250 MHz, making it suitable for usage in both hybrid and standard multi-bunch operation of the synchrotron ring. We demonstrate applications of pump-probe imaging at high lateral resolution, namely magnetic imaging and PEEM imaging of surface acoustic waves.

A near ambient pressure photoemission electron microscope (NAP-PEEM)

Photoemission electron microscopy (PEEM) is a powerful surface technique for dynamic imaging of surface processes while PEEM studies have been performed under ultrahigh vacuum or high vacuum conditions. Here we report on a near ambient pressure PEEM (NAP-PEEM) instrument, which enables high resolution PEEM imaging in near ambient pressure (> 1 mbar) gases over a wide temperature range (150 – 1200 K). Installed with an electron gun near ambient pressure low energy electron microscopy (NAP-LEEM) can be achieved as well. The success of this new NAP-PEEM/LEEM instrument relies on the following key design concepts. First, a two-stage accelerating electric field consisting of a low field region between sample and intermediate electrode (“nozzle”) and a high field between nozzle and objective lens. Second, a three-stage differential pumping system allowing a near ambient pressure atmosphere at the sample surface while ultrahigh vacuum maintained in the imaging lens systems. Third, a unique NAP cell with gas inlet/outlet, light illumination, sample cooling/heating, and precise sample positioning. The new technique will have important applications in surface catalysis, thin film growth, and energy conversion devices under nearly realistic working conditions.

Spatiotemporal Imaging of Surface Plasmons Using Two-Color Photoemission Electron Microscopy

Time-resolved photoemission electron microscopy images recorded using a combination of ultrafast 800 nm (red) and 400 nm (blue) pulses track surface plasmon polaritons launched from lithographic patterns etched into silver thin films with joint femtosecond temporal and nanometer spatial resolution. The nondegenerate two-color scheme is found to significantly enhance photoelectron yields relative to single-color approaches. This enables both an enhanced visualization of surface plasmons and a more accurate determination of surface plasmon properties compared to single-color measurements. Power-dependent photoemission yield measurements reveal that the overall signal is linear with respect to blue excitation and slightly nonlinear for analogous red excitation. A numerical model based on wave packet propagation reproduces the experimental results and rigorously establishes that the polarization fields from both laser colors and their conjugate surface plasmons account for the observed photoelectron yield enh...

Two-color multiphoton emission for comprehensive reveal of ultrafast plasmonic field distribution

We experimentally demonstrate comprehensive reveal of the ultrafast plasmonic field distribution in a bowtie nanostructure by two-color photoemission electron microscopy (PEEM). We attribute the comprehensive reveal of the field distribution to an effective opening of the two-color quantum channel in multiphoton photoemission, which leads to a dramatic reduction of the nonlinear order (from 4.07 down to 2.01) of the plasmon-assisted photoelectrons and a huge increment of the photoemission yields (typically 20-fold enhancement). Furthermore, we have found that opening extent of the quantum channel strongly related with the photoemission yields generated from one-color 400 and 800 nm laser pulse illumination, and the optimized ratio between the yields for effective opening of two-color quantum channel in our experiment is also achieved. Additionally, benefiting from the high spatial resolution of PEEM, we found there exists a large difference in the nonlinear order of two-color photoemission under the plasmonic excitation within a nanostructure, which has not been reported yet. This work introduces multicolor quantum channel photoemission into the PEEM imaging and offers new way to flexibly control the nonlinear order of the plasmon-assisted photoemission, and it will enable PEEM as a versatile tool in many potential applications.

Periodic chiral magnetic domains in single-crystal nickel nanowires

We report on experimental and computational investigations of the domain structure of ~0.2 x 0.2 x 8 {\mu}m single-crystal Ni nanowires (NWs). The Ni NWs were grown by a thermal chemical vapor deposition technique that results in highly-oriented single-crystal structures on amorphous SiOx coated Si substrates. Magnetoresistance measurements of the Ni NWs suggest the average magnetization points largely off the NW long axis at zero field. X-ray photoemission electron microscopy images show a well-defined periodic magnetization pattern along the surface of the nanowires with a period of {\lambda} = 250 nm. Finite element micromagnetic simulations reveal that an oscillatory magnetization configuration with a period closely matching experimental observation ({\lambda} = 240 nm) is obtainable at remanence. This magnetization configuration involves a periodic array of alternating chirality vortex domains distributed along the length of the NW. Vortex formation is attributable to the cubic anisotropy of the single crystal Ni NW system and its reduced structural dimensions. The periodic alternating chirality vortex state is a topologically protected metastable state, analogous to an array of 360{\deg} domain walls in a thin strip. Simulations show that other remanent states are also possible, depending on the field history. Effects of material properties and strain on the vortex pattern are investigated. It is shown that at reduced cubic anisotropy vortices are no longer stable, while negative uniaxial anisotropy and magnetoelastic effects in the presence of compressive biaxial strain contribute to vortex formation.

Valence change detection in memristive oxide based heterostructure cells by hard X-ray photoelectron emission spectroscopy

The switching mechanism of valence change resistive memory devices is widely accepted to be an ionic movement of oxygen vacancies resulting in a valence change of the metal cations. However, direct experimental proofs of valence changes in memristive devices are scarce. In this work, we have employed hard X-ray photoelectron emission microscopy (PEEM) to probe local valence changes in Pt/ZrOx/Ta memristive devices. The use of hard X-ray radiation increases the information depth, thus providing chemical information from buried layers. By extracting X-ray photoelectron spectra from different locations in the PEEM images, we show that zirconia in the active device area is reduced compared to a neighbouring region, confirming the valence change in the ZrOx film during electroforming. Furthermore, we succeeded in measuring the Ta 4f spectrum for two different resistance states on the same device. In both states, as well as outside the device region, the Ta electrode is composed of different suboxides without any metallic contribution, hinting to the formation of TaOx during the deposition of the Ta thin film. We observed a reduction of the Ta oxidation state in the low resistance state with respect to the high resistive state. This observation is contradictory to the established model, as the internal redistribution of oxygen between ZrOx and the Ta electrode during switching would lead to an oxidation of the Ta layer in the low resistance state. Instead, we have to conclude that the Ta electrode takes an active part in the switching process in our devices and that oxygen is released and reincorporated in the ZrOx/TaOx bilayer during switching. This is confirmed by the degradation of the high resistance state during endurance measurements under vacuum.

Surface Chemical Characterisation of Pyrite Exposed to Acidithiobacillus ferrooxidans and Associated Extracellular Polymeric Substances

A. ferrooxidans and their metabolic products have previously been explored as a viable alternative depressant of pyrite for froth flotation; however, the mechanism by which separation is achieved is not completely understood. Scanning electron microscopy (SEM), photoemission electron microscopy (PEEM), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and captive bubble contact angle measurements have been used to examine the surface physicochemical properties of pyrite upon exposure to A. ferrooxidans grown in HH medium at pH 1.8. C K-edge near edge X-ray absorption fine structure (NEXAFS) spectra collected from PEEM images indicate hydrophilic lipids, fatty acids and biopolymers are formed at the mineral surface during early exposure. After 168 h, the spectra indicate a shift towards protein and DNA, corresponding to an increase in cell population and biofilm formation on the surface, as observed by SEM. The Fe L-edge NEXAFS show gradual oxidation of the mineral surface from Fe(II) sulfide to Fe(III) oxyhydroxides. The oxidation of the iron species at the pyrite surface is accelerated in the presence of A. ferrooxidans and extracellular polymeric substances (EPS) as compared to HH medium controls. The surface chemical changes induced by the interaction with A. ferrooxidans show a significant decrease in surface hydrophobicity within the first 2 h of exposure. The implications of these findings are the potential use of EPS produced during early attachment of A. ferrooxidans, as a depressant for bioflotation.

Ni and CoO spin cantings induced by Fe layer in Ni/CoO/Fe/vicinal MgO(001)

Using element-resolved x-ray magnetic circular dichroism and x-ray magnetic linear dichroism measurements, we studied Ni/CoO/vicinal MgO(001) and Ni/CoO/Fe/vicinal MgO(001) systems at 350 and 78 K. Above the CoO N\'eel temperature, the Ni magnetization is fully in plane and parallel to the atomic steps in both systems due to step-induced magnetic anisotropy. Below the CoO N\'eel temperature, the CoO spins in Ni/CoO/vicinal MgO(001) are fully in plane and parallel to the atomic steps and the Ni magnetization is fully in plane and perpendicular to the atomic steps due to the 90\ifmmode^\circ\else\textdegree\fi{} Ni/CoO magnetic coupling. The CoO spins in Ni/CoO/Fe/vicinal MgO(001), however, develop an out-of-plane canted spin component in addition to the in-plane component parallel to the atomic steps. Consequently, the Ni magnetization is canted towards the out-of-plane direction by an appreciable angle. Photoemission electron microscopy imaging shows a 90\ifmmode^\circ\else\textdegree\fi{} interfacial magnetic coupling at both the Ni/CoO and the CoO/Fe interfaces and an absence of a direct interlayer coupling, showing that the Ni spin canting is due to its coupling to the canted CoO spin components which is caused by the underlying ferromagnetic Fe layer in Ni/CoO/Fe/vicinal MgO(001).

Disclosing dark mode of femtosecond plasmon with photoemission electron microscopy

The plasmonics dark mode of metal nanorings has potential in the field of e.g. sensing and surface-enhanced Raman spectroscopy (SERS). Most of the investigations on the plasmonics dark mode in a nanoring have been on far-field spectroscopy or near field simulations so far. In this paper, the near-field distribution of the femtosecond dark mode plasmon on an individual gold nanoring is mapped non-invasively using photoemission electron microscopy (PEEM). The experimental results show that strong electron emission distributions in the PEEM images under different polarization directions and wavelengths of femtosecond light are in qualitative agreement with the pattern of quadruple plasmon mode calculated using finite-difference time-domain simulation. In the meantime, it is found that there is a discrepancy in hot spot distribution between the observed PEEM image and the simulated photoelectron emission pattern, which is attributed to some possible factors, such as band structure near the Fermi level of the nanoring material and the temporal profile of femtosecond laser pulse. Real-space near-field imaging of the plasmonics dark mode provides a fundamental understanding of the near field and paves the way for further advancing the applications of dark mode in the field of e.g. sensing and SERS.