Photoemission Electron Microscopy Research Articles

Sub-100 fs Formation of Dark Excitons in Monolayer WS2.

Two-dimensional semiconducting transition metal dichalcogenides are promising materials for optoelectronic applications due to their strongly bound excitons. While bright excitons have been thoroughly scrutinized, dark excitons have been much less investigated, as they are not directly observable with far-field spectroscopy. However, with their nonzero momenta, dark excitons are significant for applications requiring long-range transport or coupling to external fields. We access such dark excitons in WS2 monolayer using transient photoemission electron microscopy with subdiffraction limited spatial resolution (75 nm) and exceptionally high temporal resolution (13 fs). Image time series of the monolayer are recorded at several different fluences. We directly observe the ultrafast formation of dark K-Λ excitons occurring within 14-50 fs and follow their subsequent picosecond decay. We distinguish exciton dynamics between the monolayer's interior and edges and conclude that the picosecond-scale evolution of dark excitations is defect-mediated while intervalley scattering is not affected by the defects.

Lifetime mapping using femtosecond time-resolved photoemission electron microscopy.

Time-resolved photoemission electron microscopy (PEEM) has established itself as a versatile experimental technique to unravel the ultrafast electron dynamics of materials with nanometer-scale resolution. However, the approach of performing PEEM-based, pixel-by-pixel lifetime mapping has not been reported thus far. Herein, we describe in detail the data pre-processing procedure and an algorithm to perform time-trace fittings of each pixel. We impose an energy cutoff for each pixel prior to spectral integration to enhance the robustness of our approach. With the energy cutoff, the energy-integrated time traces show improved statistics and lower fitting errors, thus resulting in a more accurate determination of the fit parameters, e.g., decay time constants. Our work allows us to reliably construct PEEM-based lifetime maps, which potentially shed light on the effects of local microenvironment on the ultrafast processes of the material and allow spatial distributions of lifetimes to be correlated with observables obtained from complementary microscopic techniques, hence enabling a more comprehensive characterization of the material.

Magnetic imaging of thermally switchable antiferromagnetic/ferromagnetic modulated thin films

Nanoscale magnetic patterning can lead to the formation of a variety of spin textures, depending on the intrinsic properties of the material and the microstructure. Here we report on the spin textures formed in laterally patterned antiferromagnetic (AF)/ferromagnetic (FM) thin film stripes with a period of 200 nm (100 nm FM/100 nm AF). We make use of the AF to FM phase transition in FeRh thin films at ∼100 °C, thereby creating a nanoscale pattern that is thermally switchable between AF/FM stripes and uniformly FM. A combination of spin-resolved photoemission electron microscopy, magnetic force microscopy, and magnetometry measurements allow direct nanoscale observations of the stray magnetic fields emergent from the nanopattern as well as the underlying magnetisation. Our measurements reveal pinning centres resistant to temperature cycling that govern the modulated spin-texture as well as a sub-texture consisting of grain-driven nanoscale magnetisation structure directed out of the film plane. The nanoscale magnetic structure is thus strongly influenced by the film microstructure. Signatures of exchange bias are not observed, most likely due to the small contact area between the AF and FM regions, combined with the fact that the interfaces between the damaged and undamaged regions are likely to be highly diffuse owing to the lateral scattering of incoming ions. These results show that temperature controllable spin textures can be created in FeRh thin films which could find application in domain wall, microwave, or magnonic devices.

Atomic structure of different surface terminations of polycrystalline ZnPd

The intermetallic compound ZnPd has been found to have desirable characteristics as a catalyst for the steam reforming of methanol. The understanding of the surface structure of ZnPd is important to optimize its catalytic behavior. However, due to the lack of bulk single-crystal samples and the complexity of characterizing surface properties in the available polycrystalline samples using common experimental techniques, all previous surface science studies of this compound have been performed on surface alloy samples formed through thin-film deposition. In this study, we present findings on the chemical and atomic structure of the surfaces of bulk polycrystalline ZnPd studied by a variety of complementary experimental techniques, including scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), low energy electron microscopy (LEEM), photoemission electron microscopy (PEEM), and microspot low-energy electron diffraction (μ-LEED). These experimental techniques, combined with density functional theory (DFT)-based thermodynamic calculations of surface free energy and detachment kinetics at the step edges, confirm that surfaces terminated by atomic layers composed of both Zn and Pd atoms are more stable than those terminated by only Zn or Pd layers. DFT calculations also demonstrate that the primary contribution to the tunneling current arises from Pd atoms, in agreement with the STM results. The formation of intermetallics at surfaces may contribute to the superior catalyst properties of ZnPd over Zn or Pd elemental counterparts. Published by the American Physical Society 2024

X-ray spectromicroscopy of single NiO antiferromagnetic nanoparticles

The chemical and magnetic properties of NiO nanoparticles (NP) have been studied with single-particle sensitivity by means of synchrotron-based, polarization-dependent X-ray absorption spectroscopy using photoemission electron microscopy around the Ni L3,2 edges. Three samples of NP in a size range of 40-120 nm were synthesized by thermal decomposition and subsequent calcination processes. The analysis of the local X-ray absorption spectra of tens of individual NP indicates a strong dependence of their Ni oxidation state with the calcination protocol of each sample. Additional electron-microscopy-based images and spectra of a few individual NP as well as other standard macroscopic data are in very good agreement with these experimental findings. These results showcase the relevance of combining standard and advanced single-particle studies to gain further insight into the understanding and control of electronic and magnetic phenomena at the nanoscale.

Characterization of a four-channel surface plasmon polaritons emitter based on a combined grooves coupling structure using photoemission electron microscopy

In various functional devices relying on femtosecond surface plasmon polaritons (SPPs), actively controlling the propagation direction of the SPPs field is crucial for designing plasmon-based functional devices. This capability is particularly valuable in optical logic circuits and optical communication systems. In this study, we employed photoemission electron microscopy (PEEM) to conduct near-field imaging of the SPPs field generated by a novel combination of wide and narrow grooves etched on a flat silver film. Experimental results revealed that femtosecond SPPs fields excited by laser pulses with different linear polarizations (±45°) can be emitted in four distinct directions, showcasing the combined groove structure's potential as a four-channel SPPs emitter. Additionally, Finite-Difference Time-Domain (FDTD) simulations were utilized to model the time evolution envelope of SPPs modes excited by groove coupling structures irradiated with laser pulses of varying polarization directions. The physical mechanism behind the combined groove structures as a four-channel SPPs emitter was thoroughly analyzed, and the experimental findings closely aligned with the FDTD simulation results. This research is of significant importance for advancing novel ultra-fast micro-nano optoelectronic devices with exceptional properties and for constructing ultra-fast information processing systems in plasmonics nanocircuits.

Polarization-dependent photoemission electron microscopy for domain imaging of inorganic and molecular materials.

Polarization-dependent photoemission electron microscopy (PD-PEEM) exploits spatial variation in the optical selection rules of materials to image domain formation and material organization on the nanoscale. In this Perspective, we discuss the mechanism of PD-PEEM that results in the observed image contrast in experiments and provide examples of a wide range of material domain structures that PD-PEEM has been able to elucidate, including molecular and polymer domains, local electronic structure and defect symmetry, (anti)ferroelectricity, and ferromagnetism. In the end, we discuss challenges and new directions that are possible with this tool for probing domain structure in materials, including investigating the formation of transient ordered states, multiferroics, and the influence of molecular and polymer order and disorder on excited state dynamics and charge transport.

Nanoscale photoemission in a plasmon focusing lens with a nanohole explored by time-of-flight photoemission electron microscopy

Propagating surface plasmon polaritons (SPPs) provide an important platform for the design of various photoelectric devices such as nanometer-scale ultrafast electron sources. Here, a high-brightness nanoscale photoelectron source in a plasmon focusing lens with a nanohole is investigated using time-of-flight photoemission electron microscopy (TOF-PEEM). By exploiting the high spatial resolution of TOF-PEEM, it is found that the photoemission was localized at the nanoscale in both x and y directions. In addition, a large multiphoton photoemission enhancement was achieved and a strong-field effect was observed due to the interplay between the SPP and the nanohole. This paper provides a new idea for the development of the high-brightness nanoscale photoelectron sources and a deep understanding of the interplay between SPP and LSP.

(Invited) Advanced Electronic Characterization of Wide Band Gap Semiconductor Defects at the Nanoscale

Wide bandgap materials such as gallium nitride (GaN) and gallium oxide (Ga2O3) are currently under development for realizing next-generation high power electronic devices beyond silicon. Vertical structures produced through homoepitaxial growth are preferred for scaling high-voltage devices, which places strict demands on the quality of bulk substrates. In particular, extended defects can propagate during epitaxial growth into the active areas of devices. Therefore, there is a need for identifying defects and determining their electronic properties at relevant nano- to micro- scale dimensions in order to disentangle their combined effects on macroscopic devices. Here, we use a combination of laser-based photoemission electron microscopy (PEEM) and electrical atomic force microscopy (AFM) methodologies to investigate the local electronic properties of surface defects in epitaxially grown GaN and β-Ga2O3. PEEM is a non-scanning electron microscopy technique where photoelectrons emitted by a surface illuminated by deep-ultraviolet light are imaged with nanometer-scale spatial resolution. The emitted photoelectrons are sensitive to the local electronic structure, making PEEM a powerful tool for evaluating electronic changes at length scales relevant for devices, and provides valuable information for identifying “killer” defects which must be minimized during device growth and processing.The difficulty in producing large, low dislocation density bulk GaN substrates has led to longstanding issues with leakage current in diodes and transistor devices. We identify surface defects with different electronic properties in GaN epitaxy on different substrates using PEEM. We observe star-shaped defects on strain-patterned GaN substrates and elongated dark patches on growth ridges on ammonothermally grown GaN substrates. In both cases, the observed electronic changes in PEEM correlate directly with local electrical AFM measurements, revealing that local changes in the electronic structure at these defects results in altered electrical conduction.In contrast, the development of large bulk Ga2O3 substrates has been more successful, yet the identification and influence of various defects on device performance is still largely unknown at this point. The topography of β-Ga2O3 epitaxy varies widely between growth methods and substrate orientations. For example, β-Ga2O3 by hydride vapor physical epitaxy lacks fine structure, yet has surface defects that regularly appear in parallel orientations. In β-Ga2O3 grown on different substrate orientations, we observe variations in the surface electronic properties using PEEM. For certain substrate orientations and growths, we find nanoscale variations in doping, while other orientations display micron-scale electronic inhomogeneity and defects. Our results demonstrate that laser-based PEEM can be a powerful tool for detecting electrically active defects, as well as evaluating the nanoscale electronic homogeneity of wide bandgap semiconductors.

High throughput observation of latent images on resist using laser-based photoemission electron microscopy

The rapid evolution of lithography technology necessitates faster pattern inspection methods. Here, we propose the use of laser-based photoemission electron microscopy (laser-PEEM) for high-throughput observation of latent images on an electron beam resist. We revealed that this technique can visualize latent images as chemical contrasts, and estimated the throughput millions of times higher than those of an atomic force microscope. Moreover, we estimated that throughput tens of thousands of times higher than a single-beam scanning electron microscope is achievable for post-developed resist patterns. This breakthrough highlights the potential of laser-PEEM to revolutionize a high-throughput lithographic pattern inspection in semiconductor manufacturing.

All-Optical Control of Ultrafast Switching between the Hybridized Plasmonic Fields of Au Nanorod Dimer in fs-nm Scale with Dispersed Femtosecond Laser.

With the increasing demand for ultrafast communication and information processing in future optical chips, arbitrary manipulation of electromagnetic fields in the femtosecond-nanometer spatiotemporal scale has attracted great attention in integrated optics. Manipulation of the nanoscale light field in the real femtosecond temporal domain is challenging work. Here, we have demonstrated all-optical control of ultrafast switching between the hybridized plasmonic fields of a Au nanorod dimer in the fs-nm scale using a dispersed femtosecond laser and revealed the transformation process with ultrahigh spatiotemporal resolved technology via the combination of a pump-probe technique and photoemission electron microscopy (PEEM). The results show that we can actively and coherently control the transformation sequence and time (with the shortest temporal interval of around 15 fs) of the two hybridized modes in the Au nanorod dimer by tuning the dispersion of the laser pulse. The nanoscale light manipulation achieved by all-optical control may contribute to the design of high-speed miniaturized signal-processing systems.

OsO2 as the Contrast-Generating Chemical Species of Osmium-Stained Biological Tissues in Electron Microscopy.

Electron imaging of biological samples stained with heavy metals has enabled visualization of subcellular structures critical in chemical-, structural-, and neuro-biology. In particular, osmium tetroxide (OsO4) has been widely adopted for selective lipid imaging. Despite the ubiquity of its use, the osmium speciation in lipid membranes and the process for contrast generation in electron microscopy (EM) have continued to be open questions, limiting efforts to improve staining protocols and therefore high-resolution nanoscale imaging of biological samples. Following our recent success using photoemission electron microscopy (PEEM) to image mouse brain tissues with synaptic resolution, we have used PEEM to determine the nanoscale electronic structure of Os-stained biological samples. Os(IV), in the form of OsO2, generates nanoaggregates in lipid membranes, leading to a strong spatial variation in the electronic structure and electron density of states. OsO2 has a metallic electronic structure that drastically increases the electron density of states near the Fermi level. Depositing metallic OsO2 in lipid membranes allows for strongly enhanced EM signals and conductivity of biological materials. The identification of the chemical species and understanding of the membrane contrast mechanism of Os-stained biological specimens provides a new opportunity for the development of staining protocols for high-resolution, high-contrast EM imaging.

Acoustic phonon excitation in gold probed by time-resolved photoemission electron microscopy.

Electron-phonon coupling is an important energy transfer mechanism in solids after ultrafast laser excitation. In this study, we present an extreme ultraviolet (EUV) and infrared (IR) pump-probe photoemission experiment to investigate the electron-phonon coupling in nonequilibrium gold. The energy of IR-laser-emitted photoelectrons is shifted due to the EUV photoemission and oscillates with a ∼4THz frequency. Such oscillation is considered as the effective excitation of the longitudinal acoustic phonon mode in gold through the spectral-dependent electron-phonon coupling. Our study showcases the capability of time-resolved photoemission electron microscopy to monitor the non-equilibrium lattice vibrations with ultrahigh spatial and temporal resolution.

Highly sensitive electron-beam-induced X-ray detection from liquid using SiNx membrane ultra-thinned by gas cluster ion beams

We aimed to improve the detection sensitivity for liquid measurement by developing an ultrathin photoelectron transmission window (SiNx membrane) for liquid cells via X-ray photoelectron spectroscopy or X-ray photoelectron emission microscopy at an ultrahigh vacuum. The membrane using gas-cluster ion beams (GCIB) was thinned, and its burst pressure was compared with those of membranes thinned with atomic 400 eV Ar+ ions. The SiNx membranes thinned by GCIB had approximately 2.5 times higher burst pressure than Ar+ ions. In addition, the improved sensitivity of the characteristic X-ray from liquid water induced by low-energy electrons was investigated. With the use of the 4.5 nm-thick SiNx membrane etched by GCIB, the X-ray intensity became 1.6 times higher than those of the 11 nm-thick pristine membrane at the electron beam (EB) energy of 1.5 keV. This result showed a good agreement with Monte Carlo simulation results of the EB-induced X-ray emission from liquid water beneath the SiNx membrane.

Electronic structure orientation as a map of in-plane antiferroelectricity in β'-In2Se3.

Antiferroelectric (AFE) materials are excellent candidates for sensors, capacitors, and data storage due to their electrical switchability and high-energy storage capacity. However, imaging the nanoscale landscape of AFE domains is notoriously inaccessible, which has hindered development and intentional tuning of AFE materials. Here, we demonstrate that polarization-dependent photoemission electron microscopy can resolve the arrangement and orientation of in-plane AFE domains on the nanoscale, despite the absence of a net lattice polarization. Through direct determination of electronic transition orientations and analysis of domain boundary constraints, we establish that antiferroelectricity in β'-In2Se3 is a robust property from the scale of tens of nanometers to tens of micrometers. Ultimately, the method for imaging AFE domain organization presented here opens the door to investigations of the influence of domain formation and orientation on charge transport and dynamics.

Charge-exchange-driven interfacial antiferromagnetic ground state in La0.8Sr0.2MnO3 ultrathin films

The evolution of the magnetic ground state of ultrathin 0–10 unit cells (uc) thick La0.8Sr0.2MnO3 films interfaced to an antiferromagnetic La0.45Sr0.55MnO3/SrTiO3(001) buffer layer was investigated with x-ray photoemission electron microscopy. For 0–3 uc La0.8Sr0.2MnO3, we observe antiferromagnetic domains but no ferromagnetic contrast, showing that nominally ferromagnetic La0.8Sr0.2MnO3 adopts the antiferromagnetic ground state of the buffer layer. For larger thicknesses, ferromagnetic domains emerge, confirming that the additional layers revert to the ferromagnetic ground state. We also observe a drastic increase in the complexity of the domain configuration between 3 and 5 uc, which we attribute to competing magnetic and electronic ground states in the system. We attribute the interfacial modified magnetic ground state to charge sharing at the interface due to the chemical potential mismatch, which leads to hole doping at the La0.8Sr0.2MnO3 interface. The present work sheds light on the impact of charge sharing at the interface of complex oxide materials, in particular on the magnetic and electronic states, and presents a strategy for modulating the electronic ground state properties at metallic interfaces.

Imaging Thermally Fluctuating Néel Vectors in van der Waals Antiferromagnet NiPS3.

Studying antiferromagnetic domains is essential for fundamental physics and potential spintronics applications. Despite their importance, few systematic studies have been performed on antiferromagnet (AFM) domains with high spatial resolution in van der Waals (vdW) materials, and direct probing of the Néel vectors remains challenging. In this work, we found multidomain states in the vdW AFM NiPS3, a material extensively investigated for its unique magnetic exciton. We employed photoemission electron microscopy combined with the X-ray magnetic linear dichroism (XMLD-PEEM) to image the NiPS3's magnetic structure. The nanometer-spatial resolution of XMLD-PEEM allows us to determine local Néel vector orientations and discover thermally fluctuating Néel vectors that are independent of the crystal symmetry even at 65 K, well below the TN of 155 K. We demonstrate that an in-plane orbital moment of the Ni ion is responsible for the weak magnetocrystalline anisotropy. The observed thermal fluctuations of the antiferromagnetic domains may explain the broadening of magnetic exciton peaks at higher temperatures.

Spatially resolved work-function manipulation of azobenzene-functionalized self-assembled monolayers by optical stimulation

Strongly differing static dipole moments of the trans and cis isomers of photochromic azobenzene allow for optical switching of the work function of azobenzene-functionalized self-assembled monolayers (SAMs). We apply these properties in a fundamental experiment to manipulate the area size of the switched SAM. Azobenzene molecules were excited by ultraviolet laser illumination, and the transient isomerization profile of the SAM was spatially resolved recording photoemission electron microscopy images. Thereby, we demonstrate the spatial tuning of the SAM's work function and discuss the role of the laser spot profile in generating sharp edges or gradual changes of the work function.

Exploring the Mineral Composition, Structure, and Function of a Freshwater Bivalve Adhesive

AbstractEtheria elliptica is a freshwater oyster that produces an adhesive that hardens underwater, allowing them to attach to other oysters, rocks, wood, and other natural substrates, producing complex oyster bed communities. Despite the importance of the adhesive in oyster bed formation, little is known about its chemical composition, structure, or mechanical properties. Through a combination of scanning electron microscopy (SEM), x‐ray photoemission electron microscopy (X‐PEEM), infrared (IR) spectroscopy, microhardness testing, and nanoindentation, it is found that the oyster adhesive consists entirely of aragonite nanoparticles that clump together into crystallites of varied shape, size, and orientation. The crystallite clumping density varies from the exterior of the adhesive to the interior, with the adhesive closest to the prismatic layer generally being denser. With this variation in density comes a variation in material properties with the exterior adhesive layer being softer and more flexible and the interior adhesive layer being harder and less elastic, or more similar to the prismatic and nacre shell structures produced by the E. elliptica. The resultant material exhibits varied materials properties, despite similar mineral composition.

The fate of graphene on copper: Intercalation / de-intercalation processes and the role of silicon

Intercalation and de-intercalation processes below graphene (g) grown by chemical vapor deposition (CVD) on Cu foils (g/Cu) were investigated in a combined low-energy electron microscopy and X-ray photoemission electron microscopy study. Exposure of g/Cu to air induces oxygen and water intercalation which can be removed by annealing in vacuum leading to clean and well-ordered graphene on Cu. However, prolonged air exposure leads to intercalation of large amounts of oxygen, most likely inducing the formation of copper oxides. If sufficient intercalated oxygen remains at the interface when exceeding 320 °C, graphene is oxidized and burned off. Cu foils can be loaded with silicon on purpose during foil pre-treatment or accidently during long growth time when applying high temperatures at elevated H2 pressure inducing the reactive removal of Si species from the quartz reactor wall. Due to the dissolution of Si in the Cu bulk, the Si surface concentration remains below detection limit and graphene of equal crystalline quality is grown as if the Cu foil was silicon-free. However, oxygen intercalation underneath graphene on Si-containing Cu foils can induce Si segregation towards the surface and formation of intercalated silica without attacking the covering graphene. Even at high temperatures, segregating Si acts as oxygen scavenger so that graphene resists oxidation. The observed effect explains the usefulness of certain synthesis protocols and paves the way towards large-scale fabrication of electronically decoupled graphene. The effect can be used to immobilize adsorbing oxygen at the interface and image the initial steps of intercalation below graphene in-situ.