Photoemission Electron Microscopy Research Articles

Few-cycle pulse compression and white light generation in cascaded multipass cells.

We report supercontinuum generation and pulse compression in two stacked multipass cells based on dielectric mirrors. The 230fs pulses at 1 MHz containing 12 µJ are compressed by a factor of 33 down to 7fs, corresponding to 1.0GW peak power and overall transmission of 84%. The source is particularly interesting for such applications as time-resolved angle-resolved photoemission spectroscopy (ARPES), photoemission electron microscopy, and nonlinear spectroscopy.

Quantum efficiency, photoemission energy spectra, and mean transverse energy of ultrananocrystalline diamond photocathode

The quantum efficiency and mean transverse energy of electrons emitted from a cathode determine the quality of beams generated from photoinjectors. The nitrogen-incorporated ultrananocrystalline diamond, (N)UNCD, is a new class of robust semiconductor photocathodes, which has been considered in photoinjectors for high peak current extraction. In this work, we measure the spectral response in quantum efficiency, photoemission energy spectra, and mean transverse energy of the (N)UNCD photocathode using a photoemission electron microscope. The observed quantum efficiency was comparable to that of copper photocathodes. Photoemission spectra showed the evidence of scattering of electrons before emission. This relaxation of electrons due to scattering is also observed in the spectral response of the mean transverse energy. The mean transverse energy is limited to ∼70 meV at the threshold. We attribute this to the physical and chemical roughness of the (N)UNCD photocathode and, hence, smoother films will be required to further reduce the mean transverse energy obtained from the (N)UNCD photocathode.

Giant Magnetic Anisotropy in the Atomically Thin van der Waals Antiferromagnet FePS3

AbstractVan der Waals (vdW) magnets are an ideal platform for tailoring 2D magnetism with immense potential for spintronics applications and are intensively investigated. However, little is known about the microscopic origin of magnetic order in these antiferromagnetic systems. X‐ray photoemission electron microscopy is used to address the electronic and magnetic properties of the vdW antiferromagnet FePS3down to the monolayer. The experiments reveal a giant out‐of‐plane magnetic anisotropy of 22 meV per Fe ion, accompanied by unquenched magnetic orbital moments. Moreover, the calculations suggest that the Ising magnetism in FePS3is a visible manifestation of spin–orbit entanglement of the Fe 3delectron system.

Spatially Resolved Local Electronic Properties of 2D Lead Halide Perovskite Single Crystals Studied by X‐Ray Photoemission Electron Microscopy

Recently, research on the edge states of 2D lead halide perovskites (LHPs) has been attracting much attention. The lower‐energy edge state (LES) is believed to provide an efficient pathway for the dissociation of photoexcited excitons. However, the mechanism of the LES formation remains controversial, and studies that establish precisely the local electronic properties are lacking. Herein, the first study of spatially resolved electronic structures in 2D LHP single‐crystal flakes by X‐ray photoemission electron microscopy is presented, specifically identifying the contribution from the edge area. The results show that blueshifts occur in the Pb 5d core‐level peaks at the edge area compared to the interior area with much less difference in I 4d core‐level peaks. The shift becomes more pronounced as n varies from 1 to 3 (≈0.2–1.0 eV). This phenomenon is attributed to the surface restructuring of the edge area induced by the release of mechanical strain through lattice expansion. This work provides an important reference on the origin of the LES of 2D LHPs and is beneficial for future optoelectronic device applications.

CVD Grown Tungsten Oxide for Low Temperature Hydrogen Sensing: Tuning Surface Characteristics via Materials Processing for Sensing Applications.

The intrinsic properties of semiconducting oxides having nanostructured morphology are highly appealing for gas sensing. In this study, the fabrication of nanostructured WO3 thin films with promising surface characteristics for hydrogen (H2 ) gas sensing applications is accomplished. This is enabled by developing a chemical vapor deposition (CVD) process employing a new and volatile tungsten precursor bis(diisopropylamido)-bis(tert-butylimido)-tungsten(VI), [W(Nt Bu)2 (Ni Pr2 )2 ]. The as-grown nanostructured WO3 layers are thoroughly analyzed. Particular attention is paid to stoichiometry, surface characteristics, and morphology, all of which strongly influence the gas-sensing potential of WO3 . Synchrotron-based ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), X-ray photoelectron emission microscopy (XPEEM), low-energy electron microscopy (LEEM) and 4-point van der Pauw (vdP) technique made it possible to analyze the surface chemistry and structural uniformity with a spatially resolved insight into the chemical, electronic and electrical properties. The WO3 layer is employed as a hydrogen (H2 ) sensor within interdigitated mini-mobile sensor architecture capable of working using a standard computer's 5 V 1-wirebus connection. The sensor shows remarkable sensitivity toward H2 . The high, robust, and repeatable sensor response (S) is attributed to the homogenous distribution of the W5+ oxidation state and associated oxygen vacancies, as shown by synchrotron-based UPS, XPS, and XPEEM analysis.

Endotaxial Intergrowth of Copper Telluride in GeTe-Rich Germanium Antimony Tellurides Leads to High Thermoelectric Performance

In composite materials with nominal compositions Cu2GexSb2Tex+4 (11 ≤ x ≤ 18, i.e., between Cu6.7Ge36.7Sb6.7Te50 and Cu4.5Ge40.9Sb4.5Te50), precipitates consisting of copper tellurides are endotaxially intergrown in a matrix of Cu-doped germanium antimony tellurides. The precipitates as well as the matrix material undergo various phase transitions as shown by temperature-dependent X-ray diffraction and X-ray absorption contrast imaging. Eventually, the precipitates dissolve in the matrix at temperatures exceeding 580 °C. The temperature-dependent behavior was also traced by photoemission electron microscopy up to 460 °C. At high temperatures, the thermoelectric properties are superior to those of pure germanium antimony tellurides obtained by comparable syntheses; a maximal zT value of 1.83 for Cu2Ge16Sb2Te20 is reached at 500 °C. The application of an effective mass model reveals optimal charge carrier concentrations for all three compositions investigated. The p-type Cu2Ge16Sb2Te20 material was used in combination with PbTe:In (n-type) to construct a thermoelectric module. Concludingly, the measurement of the Hall effect that suggests no significant changes in Cu-doping levels of the matrix with temperature application of grain boundary optimization and a temperature-induced reset of the microstructure are proposed as strategies for overcoming material degradation upon applying electrical currents.

(Invited) Imaging and Measuring Electronic Materials

Photoemission (PES) has been an essential metrology in the field of materials physics for over 30 years. The electronic band structure of a solid is a fundamental property which defines how a material will be used and integrated into device form. Topographic features and defects can impact electronic structure, therefore influence extrinsic properties when integrated in device form. Developing metrology that is sensitive to electronic properties at these length scales is desired. Here, we apply lab-based photoemission electron microscopy (PEEM) to gain insight in key structure – function relationships.Controlling doping during growth of semiconductors is critical as it has impact on device engineering – whether to form components in logical circuits or regions for Ohmic contact formation. For some semiconductor nanowires (NW), the doping density is varied along the long axis of the NW and knowing where the transition and its strength between doping regimes is important for device engineers. Here, we have imaged GaN NW grown with n-doped and n++-doped regions of approximately 6 μm in length and 200 nm in diameter that have been dispersed on a silicon substrate. Using a deep ultraviolet photon source, we observe an inflected change in the contrast along the long axis of the NWs (about 2 μm from one end) which we attribute to the n-n++ interface. This inflection is about 100 nm wide, and provides insight into the space-charge region formed at this electronic interface.We will also present on expanding our lab-based PEEM capability with integration to a tunable laser-based deep UV system (80 MHz, 140 fs). This added capability will provide additional imaging sensitivity to polar semiconductors such as III-nitrides. We will present some initial results with this system currently being commissioned on device-grade GaN and β-Ga2O3.

Resolving Chemical and Spatial Heterogeneities at Complex Electrochemical Interfaces in Li Ion Batteries

The development of cheap, high energy density electrode materials for Li ion batteries (LIBs) is critical to reaching net zero emissions targets and mitigating climate change. To this end, Ni- and Mn-rich transition metal oxide cathodes simultaneously offer high energy densities as well as cost effective solutions for LIBs. Despite the immense potential of these materials, both Ni- and Mn-rich cathodes suffer from severe interfacial instabilities that lead to crystallographic rearrangement of the active material surface, transition metal dissolution, and the formation of a cathode electrolyte interphase (CEI) layer on the composite surface during electrochemical cycling. While changes in the crystallographic structure of these materials are readily detected with diffraction-based methods, probing the chemistry associated with transition metal dissolution and characterizing the disordered, heterogeneous CEI layer have proven to be substantially more challenging. In this talk, I will discuss our efforts to use a combination of solid-state nuclear magnetic resonance (SSNMR) spectroscopy and X-ray photoemission electron microscopy (XPEEM) to provide information on the CEI deposited on composite cathode films with high chemical and spatial resolution (on the order of nanometers). In situ and operando magnetic resonance spectroscopies are used to correlate specific cathode degradation modes with electrolyte oxidation events. The final properties of the resulting CEI architectures are evaluated in the context of battery performance, where poor capacity retention and high interfacial resistance in LIBs is related to specific surface moieties on the cathode composite.

Near‐Field Plasmons Imaging and Energy Analysis on Bi2Te3 Film

AbstractTopological materials have great potential in lossless electronic transportation. Bi2Te3 as three‐dimensional (3D) topological insulator is predicted to exhibit many amazing electromagnetic characteristics, which are widely studied all over the world. However, current research available on Bi2Te3 materials is not adequate, and a comprehensive understanding of its properties is necessary. In this work, near‐field plasmon imaging based on Bi2Te3 nanostructures and the kinetic energy spectra of emitted electrons from Bi2Te3 film by using photoemission electron microscope instruments is studied. Under UV laser excitation of 273 nm, the electrons can be excited from the trap states in bandgap, which provides a feasible idea for observing the trap energy levels of Bi2Te3 materials. Moreover, the imaging applications of nanostructures in Bi2Te3 film based on their polarization‐dependence and wavelength‐dependence properties are also demonstrated, providing a more degrees of freedom for imaging, and can realize multidimensional and multi‐multiplexing imaging technology of topological materials. This work provides a novel method to enrich the Bi2Te3 film properties from the view of near‐field plasmon imaging and kinetic energy distribution, paving the way for realizing practical applications of topological materials.

Experimental studies of electron affinity and work function from titanium on oxidised diamond (100) surfaces

Sub-monolayers of titanium were deposited onto oxidised (100) single-crystal diamond surfaces and annealed in vacuo at temperatures up to 1000 °C to find a temperature-stable termination procedure that produces a surface with Negative Electron Affinity (NEA). The samples were analysed by X-ray Photoelectron Spectroscopy, Ultraviolet Photoelectron Spectroscopy and Energy-Filtered Photoemission Electron Microscopy to determine their electron affinity and work function values. NEA values were observed on samples following annealing above 400 °C, with the largest NEA value being –0.9 eV for a sample coated with a half-monolayer of Ti annealed at 400 °C. Work function values were ∼4.5 eV for all samples annealed at temperatures between 400 and 600 °C, then rose at higher temperatures due to the loss of substantial amounts of O from the surface. Work-function maps indicated that the surface was uniform over areas 5700 μm2, suggesting that the deposition and annealing steps used are reliable methods to produce films with homogeneous surface properties.

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.

Pattern Formation in Catalytic H2 Oxidation on Rh: Zooming in by Correlative Microscopy.

Spatio-temporal nonuniformities in H2 oxidation on individual Rh(h k l) domains of a polycrystalline Rh foil were studied in the 10–6 mbar pressure range by photoemission electron microscopy (PEEM), X-ray photoemission electron microscopy (XPEEM), and low-energy electron microscopy (LEEM). The latter two were used for in situ correlative microscopy to zoom in with significantly higher lateral resolution, allowing detection of an unusual island-mediated oxygen front propagation during kinetic transitions. The origin of the island-mediated front propagation was rationalized by model calculations based on a hybrid approach of microkinetic modeling and Monte Carlo simulations.

Investigating the morphology of bulk heterojunctions by laser photoemission electron microscopy

The nanoscale morphology of bulk heterojunctions is highly important for the charge dissociation and transport in organic solar cells and ultimately defines the performance of the cell. The visualization of this nano-morphology in terms of domain size and polymer orientation in a fast and straightforward way is therefore of great interest to evaluate the suitability of a film for efficient solar cells. Here, we demonstrate that the morphology of different blends of poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) can be imaged and analyzed by employing photoemission electron microscopy.

Flexible manipulation of plasmon dephasing time via the adjustable Fano asymmetric dimer

It is highly desirable to flexibly and actively manipulate the dephasing time of a plasmon in many potential applications; however, this remains a challenge. In this work, by using femtosecond time-resolved photoemission electron microscopy, we experimentally demonstrated that the Fano resonance mode in the asymmetric nanorod dimer can greatly extend the dephasing time of a femtosecond plasmon, whereas the non-Fano resonance results in a smaller dephasing time due to the large radiative damping, and flexible manipulation of the dephasing time can be realized by adjusting one of the nanorods in the Fano asymmetric dimer. Interestingly, it was found that plasmon resonance wavelengths both appeared red-shifted as the length of the upper or lower nanorods increased individually, but the dephasing time varied. Furthermore, it also indicated that the dephasing time can be prolonged with a smaller ascending rate by increasing the length of both the nanorods simultaneously while keeping the dimer asymmetry. Meanwhile, the roles of radiative and nonradiative damping in dephasing time are unveiled in the process of nanorod length variation. These results are well supported by numerical simulations and calculations.

High-throughput analysis of magnetic phase transition by combining table-top sputtering, photoemission electron microscopy, and Landau theory

High-throughput analysis of magnetic phase transition by combining table-top sputtering, photoemission electron microscopy, and Landau theory

Spectroscopic imaging of photoexcited states at a polycrystalline copper metal surface via two-photon photoelectron emission microscopy

The local photoexcited surface states of a polycrystalline-copper (Cu) substrate were observed via two-photon photoelectron emission microscopy (2P-PEEM) using a femtosecond laser. The 2P-PEEM results revealed local distributions of the photoexcited electrons, strongly depending on the crystal domains. With the energy-filtered images, the local 2P-photoemission spectra were selectively obtained for (111)- and (110)/(100)-crystal domains. Furthermore, the 2P-PEEM could visualize propagating Cu surface plasmon polaritons with wavevector of 7.61 × 106 m−1. These results indicate that the 2P-PEEM technique is a promising tool for studying the distribution of nonequivalent photoexcited surface states for various inhomogeneous functional nanomaterials.

Assessment of polarization-related band modulation at graphene/Mn-doped BiFeO3 interfaces by photoemission electron microscopy

Recently, ferroelectric semiconductors has become a subject of interest with regard to potential applications in novel electronic and opto-electric devices. One of the most important aspects of employing these materials is band modulation based on spontaneous polarization to generate polarization charges acting as quasi-dopants at metal/ferroelectric and ferroelectric/ferroelectric interfaces. The present study fabricated graphene/Mn-doped BiFeO3 (BFMO)/SrRuO3/SrTiO3(001) capacitor structures with the BFMO having either upward or downward polarization. Band modulation at the graphene/BFMO interface as a result of polarization charges was evaluated using photoemission electron microscopy on the BL17SU beamline at the SPring-8 facility, Japan. The chemical shifts observed in Bi 4f and C 1s XPS spectra indicated that positive (negative) polarization charges acted as quasi-dopants for electron (hole) doping of the BFMO and graphene.

Resolving the Effect of Oxygen Vacancies on Co Nanostructures Using Soft XAS/X-PEEM.

Improving both the extent of metallic Co nanoparticle (Co NP) formation and their stability is necessary to ensure good catalytic performance, particularly for Fischer–Tropsch synthesis (FTS). Here, we observe how the presence of surface oxygen vacancies (Ovac) on TiO2 can readily reduce individual Co3O4 NPs directly into CoO/Co0 in the freshly prepared sample by using a combination of X-ray photoemission electron microscopy (X-PEEM) coupled with soft X-ray absorption spectroscopy. The Ovac are particularly good at reducing the edge of the NPs as opposed to their center, leading to smaller particles being more reduced than larger ones. We then show how further reduction (and Ovac consumption) is achieved during heating in H2/syngas (H2 + CO) and reveal that Ovac also prevents total reoxidation of Co NPs in syngas, particularly the smallest (∼8 nm) particles, thus maintaining the presence of metallic Co, potentially improving catalyst performance.

Two-color laser PEEM imaging of horizontal and vertical components of femtosecond surface plasmon polaritons

Explicit visualization of different components of surface plasmon polaritons (SPPs) propagating at dielectric/metal interfaces is crucial in offering chances for the detailed design and control of the functionalities of plasmonic nanodevices in the future. Here, we reported independent imaging of the vertical and horizontal components of SPPs launched from a rectangular trench in the gold film by a 400-nm laser-assisted near-infrared (NIR) femtosecond laser time-resolved photoemission electron microscopy (TR-PEEM). The experiments demonstrate that distinct imaging of different components of SPPs field can be easily achieved by introducing the 400-nm laser. It can circumvent the risk of sample damage and information loss of excited SPPs field that is generally confronted in the usual NIR laser TR-PEEM scheme. The underlying mechanism for realizing distinct imaging of different components of the SPPs field with two-color PEEM is revealed via measuring the double logarithmic dependence of photoemission yield with the 800-nm and 400-nm pulse powers of different polarizations. Moreover, it is found that the PEEM image quality of the vertical and horizontal components of the SPPs field is nearly independent of the 400-nm pulse polarization. These results pave a way for SPPs-based applications and offer a possible solution for drawing a space–time field of SPPs in three dimensions.

Layer-Dependent Magnetic Domains in Atomically Thin Fe5GeTe2.

Magnetic domain formation in two-dimensional (2D) materials gives perspectives into the fundamental origins of 2D magnetism and also motivates the development of advanced spintronics devices. However, the characterization of magnetic domains in atomically thin van der Waals (vdW) flakes remains challenging. Here, we employ X-ray photoemission electron microscopy (XPEEM) to perform layer-resolved imaging of the domain structures in the itinerant vdW ferromagnet Fe5GeTe2 which shows near room temperature bulk ferromagnetism and a weak perpendicular magnetic anisotropy (PMA). In the bulk limit, we observe the well-known labyrinth-type domains. Thinner flakes, on the other hand, are characterized by increasingly fragmented domains. While PMA is a characteristic property of Fe5GeTe2, we observe a spin-reorientation transition with the spins canting in-plane for flakes thinner than six layers. Notably, a bubble phase emerges in four-layer flakes. This thickness dependence, which clearly deviates from the single-domain behavior observed in other 2D magnetic materials, demonstrates the exciting prospect of stabilizing complex spin textures in 2D vdW magnets at relatively high temperatures.