Photoemission Electron Microscopy Research Articles

Edge states in plasmonic meta-arrays

Abstract Photonic edge states provide a novel platform to control and enhance light–matter interactions. Recently, it becomes increasing popular to generate such localized states using the bulk-edge correspondence of topological photonic crystals. While the topological approach is elegant, the design and fabrication of these complex photonic topological crystals is tedious. Here, we report a simple and effective strategy to construct and steer photonic edge state in a plasmonic meta-array, which just requires a small number of plasmonic nanoparticles to form a simple lattice. To demonstrate the idea, different lattice configurations, including square, triangular, and honeycomb lattices of meta-arrays, are fabricated and measured by using an ultrahigh spatial resolution photoemission electron microscopy. The properties of edge states depend on the geometric details such as the row and column number of the lattice, as well as the gap distance between the particles. Moreover, numerical simulations show that the excited edge states can be used for the generation of the quantum entanglement. This work not only provides a new platform for the study of nanoscale photonic devices, but also open a new way for the fundamental study of nanophotonics based on edge states.

(Invited) The Effect of Anion Adsorption on Surface-Alloying during Underpotential Deposition

The spontaneous surface alloying during deposition is an unexplored route for developing new materials with a great potential for applications in catalysis, electronics, sensing and photonics. Although the surface-stress driven alloying has been theoretically predicted and explored on a fundamental level in the Ultra High Vacuum (UHV)1, very little of that knowledge has been employed in practice.The electrochemically controlled surface-alloying has not been understood well, although it was reported in many systems. The formation of UPD monolayer in the presence of specifically adsorbing ions is often accompanied by competitive adsorption, changes of the kinetics and energetics of nucleation and growth, and the co-adsorption that could stabilise new stages and phases of growth2,3. The effect of anion adsorption on surface alloying kinetics and energetics can significantly affect alloys’ properties and their (electro)chemical reactivity.Here in this work, we focused on systems, Pb UPD on Au(111) and Pb UPD on Cu(100), in which surface alloying has been reported in the past4-8. Pb UPD on Au(111) in perchlorate solution has been a well-studied system by a range of surface techniques4,5. However, very little is known about the effect of halide adsorption in this system. In the perchlorate solutions, in-situ surface stress relaxation4,5 and the in-situ STM4 surface changes unambiguously demonstrated changes associated with the surface alloying. Pb UPD on Cu(100) is another system where the formation of Pb3Cu4 ordered surface alloy is reported and shown to self-assemble in the nanoribbons pattern in the presence of chloride6-8. The studies on those UPD systems aimed to understand the effect of strong halides adsorption (chloride and bromide) on the electrochemical behaviour and surface alloying. Both chloride and bromide form well-ordered anion adlayers on Au and Cu while they differ in the strength of their interaction with the substrate and UPD metal layer. The studies included the electrochemical methods such as the polarisation at fixed sub-ML coverages of Pb over an extended period and following the changes in electrochemical behaviour. In addition to that, we investigated some of the structural and compositional changes by surface techniques such as X-ray Photoelectron Spectroscopy (XPS), Ultraviolet Photoelectron Spectroscopy (UPS) Energy-Filtered Photoemission Electron Microscopy (EF-PEEM) and Work Function (WF) mapping9.

Unveiling nano-scaled chemical inhomogeneity impacts on corrosion of Ce-modified 2507 super-duplex stainless steels

The widely used stainless steels and their deformed variants are anticorrosive in ambient conditions due to passivation layers composed of chromium oxides. Conventionally, corrosion and erosion of the steels are attributed to the breakdown of such layers but seldomly to the origin that depends on surface heterogeneity at the microscopic level. In this work, the nanometer-scaled chemical heterogeneity at the surface unveiled via spectro-microscopy and chemometric analysis unexpectedly dominates the breakdown and corrosion behavior of the cold-rolled Ce-modified 2507 super-duplex stainless steels (SDSS) over its hot-deformed counterpart. Though relatively uniformly covered by a native Cr2O3 layer revealed by X-ray photoemission electron microscopy, the cold-rolled SDSS behaved poorly in passivity because of locally distributed Fe3+ rich nano-islands over the Fe/Cr oxide layer. This atomic-level knowledge provides a deep understanding of corrosion of stainless steel and is expected to benefit corrosion controls of similar high-alloyed metals.

Microscopic Study of the Spinodal Decomposition of Supported Eutectic Droplets During Cooling: PtGe/Ge{110}.

We embarked on an in situ low-energy electron microscopy, photo-electron emission microscopy, and selected area low-energy electron diffraction study during the cooling of huge eutectic droplets through the critical stages of the eutectic transition. On this journey through uncharted waters, we revealed an expected initial shrinking of the exposed area of the droplet, followed by an unanticipated expansion. We attribute this behavior to an initial fast amorphization of the interface between the droplet and surface, followed by the recrystallization of Ge expelled from the droplet at the interface. As a major surprise, we discovered the emergence of extensive “spaghetti”-like patterns, which are rationalized in terms of parallel Ge ripples oriented along, mainly, [−554] and [−55–4] directions. They emerge during spinodal decomposition when passing the eutectic temperature of the system. Their sides are defined by Ge{111} and Ge{11–1} vicinals covered with Pt-modified (√3 × √3) superstructures. The distance between adjacent ripples is about 18 nm.

Carrier-Transport Mechanism in Organic Antiambipolar Transistors Unveiled by Operando Photoemission Electron Microscopy.

Organic antiambipolar transistors (AATs) have partially overlapped p-n junctions. At room temperature, this p-n junction induces a negative differential transconductance in an AAT. However, the detailed carrier-transport mechanism remains unclear. Herein, an operando photoemission electron microscopy is used to tackle this issue owing to the technique's ability to visualize conductive electrons in real time during transistor operation. Notably, it is observed that when the AAT is on, a depletion layer forms at the lateral p-n junction. The visualized depletion layer shows that both p- and n-type channels have pinch-off states in the gate voltage range when the AAT is in on state. The steep potential gradient at the lateral p-n interface enhances the electron conduction from n-type to p-type semiconductor. Another significant finding is that most electrons are considered to recombine with the accumulated holes in the p-type semiconductor, affording the reduction of photoemission intensity by ≈80%. This technique provides a thorough understanding of carrier transport in AATs, further improving the device performance.

Operando observation of analog resistance change in a buried metal/oxide interface by a laser-excited photoemission electron microscope

This study demonstrated our successful visualization of the non-volatile analog resistance changes in a metal/oxide/metal sandwiched structure. The resistance change has been intensively studied and practically utilized in the IoT devices, such as resistive random access memory and resistive analog neuromorphic device. Real-time observation technique of device operation (operando observation technique) by laser-excited photoemission electron microscopy (Laser-PEEM) enabled the visualization. The operando observation showed that the signal intensity of Laser-PEEM changed over the whole of the device area with the analog resistance changes. It is sufficiently certain that the change in signal is due to the creation and repair of oxygen vacancies throughout the device area, because the signal intensity reflects the number of electrons near the Fermi level. Considering the results of this study, we conclude that the analog resistance change depends on the device structure and oxygen vacancy concentration in metal/oxide junctions, which are controllable determinants of the device operation.

Graphene decoupling through oxygen intercalation on Gr/Co and Gr/Co/Ir interfaces

We perform a density functional theory study of the effects of oxygen adsorption on the structural and electronic properties of Gr/Co(0001) and Gr/Co/Ir(111) interfaces. In both interfaces, the graphene-Co distance increases with increasing O concentration. The oxygen intercalation effectively decreases the electronic interaction, preventing the hybridization of graphene states with Co $d$ orbitals, hence (partly) restoring the typical Dirac cone of pristine graphene. In the case of graphene/Co 1ML/Ir(111), which presents a moir\'e pattern, the interplay between the O distribution and the continuous change of the graphene-Co registry can be used to tune graphene corrugation and electronic properties. The computed electronic properties are in very good agreement with previously reported angle-resolved photoemission spectroscopy and photoemission electron microscopy measurements for Gr/Co(0001).

Ferromagnetic domain wall manipulation using optically induced thermal gradients

Magnetic domain walls can be manipulated by thermal gradients. A local thermal gradient can be generated by illuminating the surface of a thin film sample with a focused femtosecond laser beam in relation to its intensity profile. We investigate the magnetic domain structures of a ferromagnetic [Co/Pt]3 multilayers under laser illumination. We create thermal gradients by either a stationary or a moving pulsed laser beam. Following the laser illumination, we use x-ray circular magnetic dichroism as a magnetic contrast mechanism in photoemission electron microscopy in order to image the magnetic domains. Our experimental results show that the domain walls drift towards the regions of higher temperature and align themselves parallel to the laser beam motion, leading to domains elongated in that direction.

XPEEM and MFM Imaging of Ferroic Materials

AbstractThe authors describe and compare two complementary techniques that are habitually used to image ferromagnetic and ferroelectric materials with sub‐micron spatial resolutions (typically 50 nm, at best 10 nm). The first technique is variable‐temperature photoemission electron microscopy with magnetic/antiferromagnetic/polar contrast from circularly/linearly polarized incident X‐rays (XPEEM). The second technique is magnetic force microscopy (MFM). Focusing mainly on the authors’ own work, but not exclusively, published/unpublished XPEEM and MFM images of ferroic domains and complex magnetic textures (involving vortices and phase separation) are presented. Highlights include the use of two XPEEM images to create 2D vector maps of in‐plane (IP) magnetization, and the use of imaging to detect electrically driven local reversals of magnetization. The brief and simple descriptions of XPEEM and MFM should be useful for beginners seeking to employ these techniques in order to understand and harness ferroic materials.

Intermolecular interaction and cooperativity in an Fe(II) spin crossover molecular thin film system

Compact domain features have been observed in spin crossover [Fe{H2B(pz)2}2(bipy)] molecular thin film systems via soft x-ray absorption spectroscopy and photoemission electron microscopy. The domains are in a mixed spin state that on average corresponds to roughly 2/3 the high spin occupation of the pure high spin state. Monte Carlo simulations support the presence of intermolecular interactions that can be described in terms of an Ising model in which interactions beyond nearest-neighbors cannot be neglected. This suggests the presence of short-range order to permit interactions between molecules beyond nearest neighbor that contribute to the formation of largely high spin state domains structure. The formation of a spin state domain structure appears to be the result of extensive cooperative effects.

Oxygen vacancy concentration as a function of cycling and polarization state in TiN/Hf0.5Zr0.5O2/TiN ferroelectric capacitors studied by x-ray photoemission electron microscopy

We have studied the field cycling behavior of microscopic TiN/Hf0.5Zr0.5O2/TiN ferroelectric capacitors using synchrotron-based soft x-ray photoemission electron microscopy. The oxygen vacancy (VO) concentration near the top TiN/Hf0.5Zr0.5O2 interface is estimated from the reduction of Hf4+ to Hf3+ as measured in the Hf 4f core level spectra. The VO concentration increases with field cycling and redistributes under the effect of the internal field due to the polarization. Upward pointing polarization slightly depletes the concentration near the top interface, whereas downward polarization causes VO drift toward the top interface. The VO redistribution after wake-up is consistent with shifts in the I–V switching peak. The Schottky barrier height for electrons decreases systematically with cycling in polarization states, reflecting the overall increase in VO.

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.

Attenuation of Photoelectron Emission by a Single Organic Layer.

We report an in situ study of the thin-film growth of cobalt-phthalocyanine on Ag(100) surfaces using photoelectron emission microscopy (PEEM) and the Anderson method. Based on the Fowler–DuBridge theory, we were able to correlate the evolution of the mean electron yield acquired with PEEM for coverages up to two molecular layers of cobalt-phthalocyanine to the global work function changes measured with the Anderson method. For coverages above two monolayers, the transients measured with the Anderson method and those obtained with PEEM show different trends. We exploit this discrepancy to determine the inelastic mean free path of the low-energy electrons while passing through the third layer of CoPc.

Spectral and polarization based imaging in deep-ultraviolet excited photoelectron microscopy.

Using photoelectron emission microscopy, nanoscale spectral imaging of atomically thin MoS2 buried between Al2O3 and SiO2 is achieved by monitoring the wavelength and polarization dependence of the photoelectron signal excited by deep-ultraviolet light. Although photons induce the photoemission, images can exhibit resolutions below the photon wavelength as electrons sense the response. To validate this concept, the dependence of photoemission yield on the wavelength and polarization of the exciting light was first measured and then compared to simulations of the optical response quantified with classical optical theory. A close correlation between experiment and theory indicates that photoemission probes the optical interaction of UV-light with the material stack directly. The utility of this probe is then demonstrated when both the spectral and polarization dependence of photoemission observe spatial variation consistent with grains and defects in buried MoS2. Taken together, these new modalities of photoelectron microscopy allow mapping of optical property variation at length scales unobtainable with conventional light-based microscopy.

Direct visualization and control of SrOx segregation on semiconducting Nb doped SrTiO3 (100) surface

We investigated how SrOx segregates on a Nb doped SrTiO3 (100) surface by in air annealing. Using atomic force and photoemission electron microscopes, we can directly visualize the morphology and the electronic phase changes with SrOx segregation. SrOx islands less than 2 micron meter in size and 1-5 unit cells thick nucleate first and grow in a labyrinth domain pattern. After prolonged annealing, SrOx forms a ~10 nm thick film. We show that the domain pattern can be controlled by introducing a surface miscut angle of SrTiO3. Additionally, the segregated SrOx has a lower work function, compared to that of SrTiO3. These results suggest that the control and tunability of SrOx segregation is applicable to the design of a new functional electronic devices in the semiconducting SrTiO3 based heterostructure.

Nanoimaging of the Edge-Dependent Optical Polarization Anisotropy of Black Phosphorus.

The electronic structure and functionality of 2D materials is highly sensitive to structural morphology, not only opening the possibility for manipulating material properties but also making predictable and reproducible functionality challenging. Black phosphorus (BP), a corrugated orthorhombic 2D material, has in-plane optical absorption anisotropy critical for applications, such as directional photonics, plasmonics, and waveguides. Here, we use polarization-dependent photoemission electron microscopy to visualize the anisotropic optical absorption of BP with 54 nm spatial resolution. We find the edges of BP flakes have a shift in their optical polarization anisotropy from the flake interior due to the 1D confinement and symmetry reduction at flake edges that alter the electronic charge distributions and transition dipole moments of edge electronic states, confirmed with first-principles calculations. These results uncover previously hidden modification of the polarization-dependent absorbance at the edges of BP, highlighting the opportunity for selective excitation of edge states of 2D materials with polarized light.

Enhanced Magnetic Order and Reversed Magnetization Induced by Strong Antiferromagnetic Coupling at Hybrid Ferromagnetic-Organic Heterojunctions.

Organic-molecular magnets based on a metal-organic framework with chemically tuned electronic and magnetic properties have been attracting tremendous attention due to their promising applications in molecular magnetic sensors, magnetic particle medicines, molecular spintronics, etc. Here, we investigated the magnetic behavior of a heterojunction comprising a ferromagnetic nickel (Ni) film and an organic semiconductor (OSC) 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) layer. Through the magneto-optical Kerr effect (MOKE), a photoemission electron microscopy (PEEM), X-ray magnetic circular dichroism (XMCD), and X-ray photoelectron spectroscopy (XPS), we found that the adsorption of F4-TCNQ on Cu(100)/Ni not only reverses the in-plane magnetization direction originally exhibited by the Ni layer but also results in enhanced magnetic ordering. Furthermore, the cyano group (CN) in adsorbed F4-TCNQ was found spin-polarized along with conspicuous charge transfer with Ni. The density functional theory (DFT) calculations suggest that the experimentally found spin polarization originates from hybridization between the CN group's π orbitals and Ni's d band. These findings signify that the hybrid states at the organic-ferromagnet interface play a key role in tailoring the magnetic behavior of interfaces. For the case of the F4-TCNQ and Ni heterojunction reported here, interface coupling is an antiferromagnetic one.

Spatiotemporal imaging of the longitudinal and transverse components of the few-cycle surface plasmon polaritons near-field in nano-femto scale by time-resolved photoemission electron microscopy

Spatiotemporal imaging of the longitudinal and transverse components of the few-cycle surface plasmon polaritons near-field in nano-femto scale by time-resolved photoemission electron microscopy

Strong suppression of graphene growth by sulfur superstructure on a nickel substrate

The formation of multilayer graphene on a nickel substrate is usually unavoidable due to the precipitation of carbon upon cooling after the growth of the single layer of graphene. We found that the growth rate of graphene on a Ni(110) surface was markedly reduced when the surface was covered by sulfur atoms arranged in a c($2\ifmmode\times\else\texttimes\fi{}2$) periodicity. An atomistic survey by scanning tunneling microscopy revealed a compression of the sulfur superstructure as a result of the growth of graphene-covered regions. Photoelectron emission microscopy showed that the growth rate of graphene is linear with time, which indicates that carbon atoms are supplied only at the growth front of graphene islands. Density functional theory calculations rationalized these experimental results and suggested a mechanism for suppressing the growth of graphene. Sulfur atoms might extend the utility of nickel substrates for the production of single-layer graphene or nanographenes.

Direct observation of a dynamical glass transition in a nanomagnetic artificial Hopfield network

Spin glasses, generally defined as disordered systems with randomized competing interactions, are a widely investigated complex system. Theoretical models describing spin glasses are broadly used in other complex systems, such as those describing brain function, error-correcting codes, or stock-market dynamics. This wide interest in spin glasses provides strong motivation to generate an artificial spin glass within the framework of artificial spin ice systems. Here, we present the experimental realization of an artificial spin glass consisting of dipolar coupled single-domain Ising-type nanomagnets arranged onto an interaction network that replicates the aspects of a Hopfield neural network. Using cryogenic x-ray photoemission electron microscopy (XPEEM), we performed temperature-dependent imaging of thermally driven moment fluctuations within these networks and observed characteristic features of a two-dimensional Ising spin glass. Specifically, the temperature dependence of the spin glass correlation function follows a power law trend predicted from theoretical models on two-dimensional spin glasses. Furthermore, we observe clear signatures of the hard to observe rugged spin glass free energy in the form of sub-aging, out of equilibrium autocorrelations and a transition from stable to unstable dynamics.