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[Shunsuke Tsuda](https://orcid.org/0000-0001-6209-8048), [Koichiro Yaji](https://orcid.org/0000-0002-0721-1316)

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[Time-of-Flight-type Photoelectron Emission Microscopy with a 10.9-eV Laser](https://mdr.nims.go.jp/datasets/75986ada-6ffe-4830-8c32-29651c5b00a1)

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e-Journal of Surface Science and Nanotechnology 22, 170–173 (2024)Time-of-Flight-type Photoelectron Emission Microscopywith a 10.9-eV LaserShunsuke Tsuda,† Koichiro YajiCenter for Basic Research on Materials, National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan† Corresponding author: TSUDA.Shunsuke@nims.go.jpReceived: 22 November, 2023; Accepted: 31 December, 2023; J-STAGE Advance Publication: 17 February, 2024; Published: 17 February, 2024We have developed a novel photoemission microscopy apparatus employing avacuum ultraviolet laser. This setup combines photoemission electron microscopy(PEEM) with a time-of-flight detector, facilitating rapid visualization of electronicstates in both real and momentum space. Achieving a spatial resolution of 70 nm,attributed to the PEEM lens system, we showcase the full band mapping of a Bi(111)single crystal film using angle-resolved photoemission spectroscopy within a shortacquisition time.Keywords Photoemission spectroscopy; Momentum microscope; Ultraviolet laser;Electronic property; Time of flightI. INTRODUCTIONPhotoelectron spectroscopy provides direct insights intothe electronic states of materials by measuring the kineticenergy of photoelectrons emitted upon irradiation with lightexceeding the work function and emission frequency at thatenergy [1]. In the context of single-crystalline samples, theemission angle furnishes information on the momentum ofelectrons in the material, giving rise to angle-resolved photo-emission spectroscopy (ARPES), a technique that has sig-nificantly advanced condensed matter physics.The advancement of ARPES technology parallels en-hancements in hemispherical photoelectron analyzers, evolv-ing rapidly over the last quarter century to achieve excep-tional energy resolution [2]. Hemispherical energy analyzers,capable of directly visualizing the band structure, proveespecially well-suited for investigating electronic propertiesand solid-state physics. However, their inherent limitationlies in measuring only a specific narrow energy range simul-taneously, discarding information from other photoelectronsand presenting challenges for samples with weak photoelec-tron signals. Conversely, the time-of-flight (ToF) analyzer, adetector for photoemission spectroscopy with a 50-year his-tory [3], detects differences in kinetic energy by measuringthe time it takes for photoelectrons to reach the detector.While highly efficient as it captures all electrons, ToF ana-lyzers necessitate a pulsed light source and have predomi-nantly been used with synchrotron radiation.In laboratory systems, pulsed-laser lights are employed forphotoelectron spectroscopy with ToF detectors. Althoughsome light sources surpass the work function’s energy, theirlow energy restricts the measurable momentum space se-verely. A recent breakthrough in a light source with a photonenergy of 10.9 eV has overcome this limitation, ushering inthe ToF option for photoelectron spectroscopy in modernlaboratory systems [4].Photoemission electron microscopy (PEEM) is a widelyutilized technique for investigating electronic and magneticproperties on solid surfaces, encompassing tasks such asimaging magnetic domains [5–7], measuring work functions[8], and studying ultra-fast carrier dynamics in semiconduc-tors [9]. Despite lacking energy analysis capability, PEEMboasts high spatial resolution [10]. Integrating a ToF analyzerinto a PEEM column (ToF-PEEM) introduces energy analy-sis to PEEM images with remarkable resolution [11], render-ing ToF-PEEM a viable spectroscopic technique. Recentadvancements include time-resolved photoemission spectros-copy [12, 13] and spin analysis using a multi-channel spindetector [14, 15].This study unveils a ToF-PEEM spectrometer coupledwith a 10.9-eV laser. This innovative spectrometer achieveshighly efficient data acquisition with exceptional spatial res-olution in photoemission microscopy. Moreover, the PEEMlens systems facilitate swift switching between real andTechnical Notee-J. Surf. Sci. Nanotechnol. 22, 170–173 (2024) | DOI: 10.1380/ejssnt.2024-005 170mailto:TSUDA.Shunsuke@nims.go.jphttps://doi.org/10.1380/ejssnt.2024-005momentum spaces. This functionality enables rapid identifi-cation of the region of interest in the sample’s real space,followed by an examination of the band structure in themomentum space.II. INSTRUMENTA. Analyzer and light sourceThe ToF-PEEM consists of six components, as illustratedin Figure 1, with laser specifications detailed for the relevantparts [16]. The system operates in two modes for acquiringphotoelectron images: the conventional PEEM measurementmode utilizing a multi-channel plate and a CMOS camera set,and the ToF mode employing a delay-line detector (DLD,Roentdek DLD40). In the conventional PEEM mode, anenergy-integrated photoelectron image is obtained, whilethe ToF mode captures an energy-resolved image instanta-neously. Consequently, a three-dimensional dataset is con-structed by adding energy to two dimensions (x, y, or kx, ky)simultaneously. The ToF drift tube is 457mm long.The laser employed is a commercial 10.9-eV (113.8-nm)device (OXIDE UV-3), utilizing the 9th harmonic of thefundamental Yb fiber laser. The fundamental is transformedinto the second harmonic (2ω = 512 nm) by the first SHGcrystal (LiB3O5). Subsequently, the second SHG crystal(CsLiB5O10) converts the second harmonic into a fourthharmonic (4ω = 256 nm). The Xe gas cell combines thetwo harmonics and the fundamental to generate the 9thharmonic (9ω = 4ω + 4ω + 1ω = 113.8 nm). With a repeti-tion frequency of 50MHz, only 1 pulse out of 32 is utilizeddue to the inability of DLDs to keep up with 50MHz. Thepulse width is ≲20 ps. The typical output power of the laseris 5 µW, and the spot size, as per literature [16], measuresapproximately 130 µm × 320µm, with a photon flux of 6.9 ×1013 photons s−1mm−2. The number of photons per pulse is1.4 × 107 photonsmm−2 pulse−1. Despite the extended pulsewidth, spatial charging effects are mitigated due to the lowphoton density. The high photon flux per second enablesextremely efficient measurements to be made. The energywidth is less than 0.1meV, ensuring suitability for high-resolution photoemission spectroscopy measurements. Thelight, horizontally, vertically, clockwise, and counterclock-wise polarized, is focused onto the sample through a concavemirror at an irradiation angle of 65° from the sample surfacenormal direction. To prevent absorption by water and oxy-gen, the laser chamber is filled with argon gas and isolatedfrom the ultrahigh vacuum by a LiF window.B. Conversion from time to energyFigure 2(a) presents a ToF-detector-captured PEEM imageof a sample featuring silver pads deposited on a siliconsubstrate. The trench width between the silver pads measures2 µm, with a period of 10 µm between the pads. The silverpad pattern is clearly discernible. The intensity profile alongthe x-direction within the yellow square in the figure isdepicted in Figure 2(b). Fitting the result by convolving astep function with a Gaussian function reveals a full width athalf maximum of 70 nm, indicative of a spatial resolution ofat least 70 nm. This value is the upper limit of the spatialresolution in this system because it implies the edge steep-ness of the Ag pads. The ToF detector used for these imagesallows the determination of photoelectron kinetic energyfrom the delay time. The process involves capturing the timedependence of the photoelectron intensity (time spectrum),applying a bias voltage to the sample concerning the analyzerto shift the spectrum, and obtaining the full-time spectrum[Figure 2(c)]. The change in the time position of the peakdue to this bias is linearly approximated, enabling the con-version of energy from the delay time. Figure 2(d) illustratesthe dependence of the peak position on the bias voltage,demonstrating an almost linear relationship. Fitting with astraight line yields a slope of 1.375 nsV−1. Given the sys-tem’s smallest time step of 64 ps, the smallest energy step isapproximately 50meV. Notably, the time resolution of theToF, rather than the electron lens system, currently dictatesthe energy resolution.Figure 1: Schematic of ToF-PEEM and laser specifications.Technical Notee-J. Surf. Sci. Nanotechnol. 22, 170–173 (2024) | DOI: 10.1380/ejssnt.2024-005 171https://doi.org/10.1380/ejssnt.2024-005III. DEMONSTRATIONAs an illustration of the ARPES measurement using theToF-PEEM system, we performed ARPES on a Bi(111)single crystal film grown on a Ge(111) substrate. The cleansurface of the substrate was achieved through several cyclesof Ar+ sputtering and annealing up to 600°C, confirmed by asharp low-energy-electron-diffraction (LEED) pattern dis-playing c(2 × 8) periodicity [17]. Subsequently, a 100-nmBi film was deposited at room temperature (RT) via molecu-lar beam epitaxy and annealed at 400K [18]. The depositionrate was monitored using a quartz microbalance, and order-liness of the Bi film was verified by a distinct (1 × 1) LEEDpattern, consistent with prior research [18]. ARPES measure-ments were performed at RT.Figure 3 presents kx–ky intensity mapping at the Fermilevel [Figure 3(a)], an E–kx map along the �M direction[Figure 3(b)], and an E–ky map along the �K direction[Figure 3(d)]. These data were acquired within a real-spacefield of view of 50µm. The energy range covered the entireobservable spectrum from Fermi energy to kinetic energy 0.At the Fermi level, we observed a circular Fermi surfacecentered at the � point and six elliptical Fermi surfacesextending to the �M direction, representing surface statesof Bi(111), consistent with prior studies [19]. Additionally,several bands emerged on the deeper binding energy side,corresponding to bulk states and continuums [20]. As evidentin Figure 3(b, c), the photoelectron intensity is observedwithin a parabola (the so-called photoelectron horizon) witha bottom at EB = 6.2 eV, aligning with previous studies [20].This example illustrates that a 10.9-eV laser facilitates meas-urements across a broad range in both momentum space andenergy direction. In the case of the Bi(111) film, the Fermisurfaces of the entire Brillouin zone and a band a few eV nearthe Fermi level could be observed [16].The acquisition time for the full band mapping depicted inFigure 3 was only 20min, allowing for swift data acquisitionfrom numerous measurement points. Furthermore, the ToF-PEEM spectrometer can be also utilized to measure thesamples before contaminated.Figure 2: (a) Photoelectron intensity mapping of the square gridpattern of Ag on the Si substrate in real space with a field of view(FoV) of 50 µm. The inset is an enlarged image of the yellow box inthe main panel. (b) Photoemission intensity profile along the x-direction inside the square in panel (a). (c) Spectral change due tobias application. (d) Relationship between energy and delay time.Figure 3: (a) ARPES intensity mapping of the Bi(111) film on theGe(111) substrate in the E–kx–ky cube. (b) Photoemission intensitymapping in a kx–ky plane at the Fermi level. (c) Photoemissionintensity mapping in an E–kx plane at the ky = 0 line. (d) Photo-emission intensity mapping in an E–ky plane at the kx = 0 line. Thesurface Brillouin zone is superimposed by yellow thin lines on panel(b). The incident light plane is parallel to �M, and p-polarized lightwas used.Technical Notee-J. Surf. Sci. Nanotechnol. 22, 170–173 (2024) | DOI: 10.1380/ejssnt.2024-005 172https://doi.org/10.1380/ejssnt.2024-005IV. SUMMARYIn summary, we constructed a ToF-PEEM using a 10.9-eVlaser, achieving a spatial resolution of 70 nm in real space.Additionally, we established an energy calibration methodand successfully conducted microscopic ARPES measure-ments on a Bi film on a Ge substrate, obtaining data acrossall measurable regions in just 20min.AcknowledgmentsThe authors thank Nils Weber for technical support in developingthe spectrometer. 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