Spatially Resolved Photoelectron Spectroscopy from Ultra-high Vacuum to Near Ambient Pressure Sample Environments
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Modern scanning photoemission microscopes use zone plates to de-magnify the X-ray beam to nanometer size allowing spatially resolved XPS analysis of materials relevant in nanotechnology. So far these microscopes have been designed to operate in the ultra-high or high vacuum environments as all XPS systems; but at the beginning of this century the dream of K. Siegbahn, the inventor of XPS, to use it in the near ambient or ambient pressure regimes became a reality. Despite the fast development and spread of these setups designed for not spatially resolved experiments, now available both as synchrotron and laboratory facilities, it took more than a decade before a similar result could be extended to photoemission microscopy. The scanning photoemission microscope at Elettra is the first instrument where near ambient pressure conditions for in operando analysis can be fulfilled. This paper shows some recent results obtained at this microscope at different sample environment conditions.
KeywordsIn operando Near ambient pressure XPS Scanning photoelectron microscopy Pt nanoparticles Oxidation
The implementation and development of photoelectron spectromicroscopy techniques was boosted by the construction of the 3rd generation synchrotron sources started almost 30 years ago. Photoelectron microscopes can be classified in two groups based on the different technological approach to microscopy they use. In the case of scanning instruments, called scanning photoelectron microscopes (SPEM), the X-ray beam is focused down to micron or submicron sized dimensions by mean of focusing optics [e.g. zone plates (ZP) or mirrors] and the sample is raster scanned in front of it; at each step of the scan the photoelectron signal is detected and processed. In the other group photoelectron emission microscopes (PEEM or X-PEEM) use a static approach: samples are fully illuminated by the X-rays and the photoelectrons entering the electron analyzer will experience an electromagnetic field which at the end will produce a magnified image of the illuminated sample area. The two approaches generate complementary microscopes with advantages and disadvantages; for instance PEEMs can hardly address samples with a high surface roughness while SPEMs are not affected by that or PEEMs can acquire images at the video rate while SPEMs are intrinsically slower .
Since the first pioneering setups all photoemission microscopes have been designed for high vacuum (HV) or ultra-high vacuum (UHV) instruments to preserve the photoelectron signal in the detection process as it happened for the instruments designed for photoelectron spectroscopy [2, 3]. In the last decades most of the efforts have been spent to improve the lateral and energy resolution of these microscopes; thanks to such improvements modern SPEMs can achieve spatial resolutions around 70 nm as well as energy resolutions better than 0.2 eV. SPEMs and XPEEMs can be found in most of the worldwide synchrotron facilities contributing to the exploration of surface and interfacial properties of technologically important materials. In this scenario the Elettra synchrotron laboratory is a unique place hosting four different photoelectron microscopes accessible to the scientific user community.
Despite it was the idea of the inventor of the X-ray photoelectron spectroscopy (XPS) Siegbahn and Siegbahn  to use it with environments close, at least, to the near ambient pressure (NAP) regime, it took almost half a century before scientists could develop the first instrument capable to perform XPS at pressures around 1 mbar (NAP–XPS). Indeed it was the beginning of this century when the first differentially pumped electron analyzers were developed and used to overcome the so called “pressure gap” . Such kind of instrumentation is now available at many synchrotron facilities and as laboratory equipment too. Despite the impressive progress in the development of these setups, it was not possible to combine NAP environments with submicron spatially resolved XPS measurements so far. Almost a decade ago, the team of the Escamicroscopy beamline at the Elettra synchrotron laboratory started to design and test innovative solutions, which allow the overcoming of the “pressure gap” limit also for SPEMs [6, 7]; one of the solutions found is a special cell (called NAP-Cell) capable to perform in operando experiments at pressures as high as 1 mbar. As will be more extensively discussed in the next section the SPEM at Elettra  is a unique microscope offering the possibility to perform experiments from the UHV to the NAP pressure regime keeping the best allowable performance.
This manuscript will present the results of two experiments performed with the SPEM at Elettra using two different pressure regimes; in the first case a UHV investigation of Pt nanoparticles created on suspended graphene (Gr) flakes will be shown while in the second one the oxidation of a polycrystalline Ru sample will be discussed. The aim is to demonstrate the flexibility of SPEM in accommodating experiments which require a different environment.
2.1 UHV and Near Ambient Pressure SPEM Setups
A schematic view of the focusing optics and photoelectron detection setups of the SPEM of the Escamicroscopy beamline at Elettra is shown in Fig. 1. The X-ray beam produced by the ring Elettra is focused on the sample using Fresnel lenses called ZP, which allow to achieve a spot diameter of ~ 130 nm. Samples can be raster scanned with respect to the X-ray microprobe and, at each step of the scan, the photoelectrons generated at the sample surface are collected and energetically selected by means of an hemispherical electron analyzer (HEA) equipped with a 48-channels electron detector . The position of the HEA with respect to the sample surface is fixed to 30°. This configuration strongly enhances the surface sensitivity of the instrument because the photoelectrons generated in the inner layers and directed towards the HEA have a longer path inside the dense matter and may not have enough kinetic energy to escape. This feature makes the SPEM more sensitive to any adsorbate present on the surface which may be a drawback, if the molecules are undesired, or a favorable condition for instance in the case of studies of catalysis. It should be noted that the typical distance between ZP and samples is just few mm; this aspect is the main reason why the technical solutions found to overcome the “pressure gap” for XPS setups can not be easily transferred to SPEMs. A SPEM can be operated in two modes: (i) imaging spectromicroscopy and (ii) microspot spectroscopy. The imaging mode maps the lateral distribution of elements by collecting photoelectrons within a selected kinetic energy window while scanning the specimen with respect to the microprobe. When the element under consideration is present in a single chemical state, the spatial variation in the contrast of the images reflects the variation of the photoelectron yield, which is a measure of the local concentration of that specific element. The microspot mode is similar to the conventional XPS, i.e. energy distribution curves are measured from the illuminated local micro-spot area.
In the UHV configuration of the SPEM samples, as shown in Fig. 1, are exposed to the same common environment which hosts the focusing optics, the HEA and other equipment; in this configuration the highest allowable pressure is about 1 × 10−5 mbar in the case of experiments where samples need to be exposed to gases.
The NAP-Cell uses small pinholes as photon-in/photoelectron-out apertures in a vacuum sealed cell, with the sample encapsulated within [10, 11]. A sketch of this setup is shown in Fig. 2 where a sectional view of the cell is shown. The reactive gases needed in the experiment are delivered into the cell through a metal flexible bellow which is part of the dosing line inside the SPEM chamber. A gas dosage system and a pressure gauge regulate the gas flow into the dosing line. An encapsulated heater, placed on the back of the sample, allows the sample temperature to be varied in the range 300–820 K. Additional electrical contacts are available for biasing the sample. The gap in the pressure between the SPEM vacuum chamber and the inner volume of the NAP-Cell is established by the impedance of the small pinholes, and makes it possible to reach NAP conditions (~ 0.1–1 mbar) inside the cell maintaining HV condition outside (10−5 mbar) which is safe for the use of a standard HEA instead of a differentially pumped one. The entire cell is scanned with respect to the X-rays, as for conventional samples. The “visible” area of samples is defined by the size and orientation of two holes and is approximately a circle of 0.4 mm diameter. The spatial and energy resolutions have no restrictions when the NAP-Cell is used instead of the UHV setup.
2.2 Sample Preparation and Experimental Conditions
The characterization of suspended thin films is one of the cases where the capabilities of the SPEM provide a unique tool for chemical and physical analysis. Novel materials such as graphene, MoS2 or WS2 can be grown in the form of very thin films with atomic thickness. When the thickness and the atomic structure are precisely controlled the lateral dimensions of such films reduce to that of micron sized flakes. With SPEMs it is possible to investigate these flakes in the suspended configuration, i.e. where no interaction with any substrate is present. This is shown in the SPEM map acquired at the C 1s (Fig. 3a) showing a Holey Carbon film partially covered by Gr flakes; the clearly visible amorphous carbon network forms holes which are empty (black areas) or covered by Gr (grey areas). Another approach is presented in panel b); here a patterned network of holes has been created on a Quantifoil film (amorphous C) and covered by Gr; the photoemission map reveals the holes which are empty (dark ones) and those which are covered by Gr. Both images demonstrate how a thin, 1–2 monolayers (ML), suspended Gr flake generates enough photoelectrons to be clearly measured.
For the Pt/Gr experiment described in this manuscript samples used were holey silicon nitride support membranes from Graphenea . The platform of these membranes is the low stress 200 nm thick amorphous Si3N4 support film on a circular 3 mm silicon frame with a 0.5 × 0.5 mm2 membrane size. The platform includes 2 µm diameter holes at every 5 µm. The Gr sheets were covering the entire Si3N4 window holey membrane leaving suspended Gr regions on top of the 2 µm holes and supported flakes outside of them. The thickness of the Gr, as declared by the manufacturing company, was a single layer. To clean the Gr samples after being introduced in the UHV sample preparation chamber they were annealed at 723 K for 12 h in order to remove light adsorbates from the surface. Pt depositions were performed in the same UHV chamber by using an electron bombardment evaporator previously calibrated. The amount of Pt deposited on the Gr samples was equivalent to a fraction of one ML.
High-resolution transmission electron microscopy (HRTEM) and high angle annular dark field-scanning TEM (HAADF-STEM) analyses were carried out on exactly the same samples by a JEOL JEM-2200FS instrument, equipped with a CEOS spherical aberration corrector for the objective lens and an in-column image filter (Ω-type), with a Schottky source (operated at 200 kV for these experiments). This instrument is part of the Electron Microscopy Facility of the Istituto Italiano di Tecnologia in Genova, Italy.
The Ru sample was a homemade polycrystalline foil prepared by pressing and mechanically polishing commercial Ru powders. Before its use in the experiments the foil was treated by several Ar ion bombardment/high temperature annealing/molecular oxygen bath cycles to remove most of the impurities.
In the first experiment a photon energy of 650 eV has been used while for the Ru oxidation it was 751 eV. The core level spectra shown in the result section have been acquired by setting the overall energy resolution of the experimental setup to 0.2 eV. For the deconvolution of the XPS spectra we used standard procedures based on Doniach-Sunjic functions convoluted with Gaussians accounting for experimental resolution and spectra broadening. The precise calibration of the binding energy (BE) of the core level spectra was defined by measuring a clean Au foil and fixing at 84.0 eV the BE of the Au 4f7/2 peak.
3 Results and Discussions
3.1 Pt nanoparticles on Vicinity Domains Within Suspended and Supported Graphene
Graphene is one of the most interesting and investigated novel materials because of its potential for a wide range of applications covering all fields of science and technology. Some of its properties, which include for instance a high electrical conductivity, an enhanced surface to volume ratio and good thermal and mechanical stabilities, make it a suitable material for designing the next generation of catalytic, electrochemical, and sensing systems [13, 14, 15, 16, 17, 18, 19, 20].
Carbon-based supports, such as nanotubes, nanoribbons or graphite, covered by Pt nanoparticles have already been demonstrated to exhibit high efficiencies for applications such as hydrogen storage, CO oxidation, oxygen reduction reaction, electrochemical sensing, and in fuel cells [13, 14, 15, 16, 17, 18, 19, 20, 21]. Since its availability, Gr interaction with Pt atoms has been investigated as this system represents a potential material for catalytic or electrochemical applications. In the majority of the studied cases the Gr flakes are supported by substrates of different materials where Gr has been grown or transferred. Due to the fact that with the nowadays available technologies only holes of small sizes can be covered with thin (one or few layers) Gr films it is extremely difficult to study the behavior and properties of suspended Gr with characterization tools different from electron microscopies. This section of the manuscript demonstrates how the scanning photoemission microscopy, combined with transmission electron microscopy is capable to investigate these particular systems.
The deposition of the Pt on the Gr flakes was performed while keeping the system at room temperature; it turned out to create nanometer sized individual particles and agglomerated ones (defined as clusters hereafter) with a different distribution on the suspended and supported Gr. An overview of the effects of the Pt is shown in Fig. 4 where a TEM analysis of the sample is presented. The HAADF-STEM image in panel a has been acquired on the suspended Gr: it shows the presence of two distinct areas, one covered with nanometer size Pt particles well separated from each other with larger particles at the boundary lines and another one where larger particles agglomerate to form clusters dispersed with a lower density. Note that the whole area is covered by Gr. The size of the small nanoparticles is in the range 1–3 nm while that of the larger ones forming clusters is 5–15 nm. A detailed statistical distribution of the islands dimensions can be found in Ref. . All the suspended Gr flakes were covered in this way. A different scenario is present outside the holes where Gr is supported by the Si3N4 substrate; map b shows, in fact, a surface almost entirely covered by the nanometer size Pt particles without a significant presence of larger Pt agglomerates if compared with the suspended case. Also this area is entirely covered by Gr. The poorer contrast in the image is generated by the attenuated transmitted TEM probe because of the presence of the Si3N4 support. The crystalline structure of the Pt nanoparticles has been revealed by HRTEM imaging as reported in panel c where several clusters are visible, some of them being clearly identified as crystalline Pt nanoparticles (see the FFT plot in panel d calculated for the selected area in image c). The same surface areas were then analyzed by the SPEM; the corresponding photoemission characterization is included in Fig. 5. The spectrum (Fig. 5a) is a survey scan acquired on the suspended Gr showing the accessible core levels and Auger lines of Pt and C with traces of Si, a typical contaminant of Gr samples. Two representative high resolution C 1s core level spectra recorded on the Pt covered surface are shown in Fig. 5b, c for the suspended and supported Gr, respectively.
In both spectra a dominating component located at 284.4 eV BE originates from sp2-hybridized graphite-like carbon atoms  and exhibit the typical asymmetric line shape at higher BE generated by many-electron interactions in the photoemission process. A weaker one at lower BE associated to the C vacancies in the Gr sheet is centred at 283.7 eV. It has been shown, in fact, [24, 25] that BE of all C atoms in the hexagons surrounding a single C vacancy undergo chemical shifts toward lower BE with respect to the C 1s peak of a perfect sheet. In order to obtain also a good deconvolution at higher BE another component at 285.3 eV BE, attributed to the presence of adventitious carbon, is needed; its contribution is more pronounced in the case of supported Gr. In the case of the supported Gr an additional component at the lower BE side of the C 1s located at 283.0 eV is necessary to have a good deconvolution. It could be attributed to Pt clusters bound to vacancies in the Gr and the resulting interaction between the Pt and the Gr. Spectra d and e of Fig. 5 are two Pt 4f7/2 spectra acquired, as for the C 1s, in the suspended and supported Gr respectively. The colored signals have been generated by measuring a clean metallic Pt foil and represent the metallic residual component in the two cases; it is centered at 71.0 eV BE in well agreement with standard values for metallic Pt. The raw data spectra appear much broader than the reference metallic Pt; their deconvolution, indeed, needs a second component at higher BE and more dominant for the smaller particles on the supported Gr corresponding to low coordinated atomic arrangements of Pt atoms. The additional component is broader because it includes all the contributions from a widely varied particle size distribution. It is known, in fact, that for nanoparticles core level shifts generated by the particle size must be considered in the analysis of such spectra . The distribution of the size of the Pt particles in the reported analysis falls in the range where the energy shifts can be in the order of 0.2–1 eV . Moreover, it has been shown that the electronic structure of the cluster can be modified both due to bond formation as well as due to strains induced in the cluster and that there is a charge transfer from the Pt cluster to the Gr support [28, 29].
3.2 In Operando Study of the Oxidation of Ru at Near Ambient Pressure
Ruthenium is a highly active catalyst and can catalyze many important reactions [30, 31, 32, 33, 34, 35]. The catalytic activity of Ru is known to change if the pressure of the gas reactants lays in the UHV regime or at higher values; the reason resides in the different activated mechanisms in the formation of Ru oxides [33, 35]. To understand the involved processes and the influence of the oxide in these reactions it is necessary to perform in operando experiments at elevated pressures. Recently, indeed, the oxidation of Ruthenium and its catalytic CO oxidation were investigated in operando with low energy electron microscopy and NAP–XPS [36, 37, 38].
Here, we investigated the oxidation process of Ru using the NAP-Cell in the SPEM combining high pressures with spatial resolution. After transferring into the NAP-Cell the sample was further cleaned by several oxidation and reduction cycles at 0.1 mbar and elevated temperatures.
The in operando oxidation of Ru was performed at 620 K with an O2 pressure of 0.1 mbar while monitoring the Ru 3d5/2 signal. The obtained XPS spectra are shown in Fig. 6a. Before introducing the oxygen the corresponding Ru 3d5/2 spectrum (clean) showed one component (blue) at a BE of 280.0 eV. This component is attributed to metallic Ruthenium . After introducing O2 with a pressure of 0.1 mbar a shoulder appeared at the high BE side of the spectrum. The deconvolution of the signal shows that the shoulder can be described by an additional component (green) shifted by 0.7 eV towards higher BE with respect to the metallic Ru. This component reveals the initial formation of the RuO2 phase . The intensity of the new component was increasing with prolonged oxygen exposure and reached saturation after about 45 min when the oxide phase accounted for about 14% of the probed volume. A quantification, according to Ref. , reveals an effective thickness of the oxide layer of about 1.4 Å which corresponds to about 0.4 layers of RuO2 . The saturation is in line with the literature data but thicker oxide layers (few nm) are described there [40, 41]. Higher oxidation states of the Ru indicated by additional signals shifted by 1.5–3.3 eV towards higher BE with respect to the metallic Ru  were not observed in our oxidation experiments. This is consistent with previous NAP–XPS investigations [36, 37] while the higher oxidation states were only observed after ex situ oxidation using harsher conditions [39, 42].
An O 1s core level spectrum is shown in Fig. 6b. It was taken subsequently to the oxidation series shown in Fig. 6a after a near ambient pressure oxygen dosing of 140 min. The broad feature between 529 and 535 eV originated from the oxygen atoms bonded to the Ru and to small amounts of contaminations like Si and C found on the surface. The two sharp signals at 538.3 and 539.4 eV can be attributed to gas phase molecular oxygen  that was visible due to the oxygen pressure of 0.1 mbar inside the NAP-Cell.
An image of the oxidized Ru surface is shown in Fig. 6c. The image was taken using the Ru 3d5/2 signal covering a spectral region from 278.9 to 282.7 eV distributed over the 48 channels of the photoelectron detector. The visible contrast arose from the topography of the sample, i.e. the orientation of the surface normal with respect to the analyzer. The features (holes) are grain boundaries as it is expected for the polycrystalline Ru pallet. To obtain the spatial distribution of the grown oxide a chemical contrast image was generated by dividing the channels containing the oxide component by the channels containing the metal (see indicated bars in Fig. 6a). This procedure also removes the topographical information. The result is shown in Fig. 6d. Here, the bright areas correspond to a higher amount of the surface oxide phase. The most prominent features with a higher amount of formed oxide are the grain boundaries on the right side of the image and the circular grain on the left side. The fact that grain boundaries can enhance the oxide formation is consistent with the description of defects as grains for the oxide growth . The inhomogeneous distribution of the amount of oxide phase on the flat areas is related to the distribution of the Ru grains and their orientation. Therefore, this is a clear indication that the surface orientation has an effect on the oxide formation. The presented data prove the necessity of spatial resolved measurements to investigate and understand the oxidation process of Ruthenium.
Scanning photoelectron microscopy has become an important tool at synchrotron laboratories for probing surface and interface properties of technologically relevant materials. Since the design of the first microscopes, occurred after the construction of the 3rd generation synchrotron sources, continuous efforts for improving not only the achievable lateral and energy resolution but also sample environment have been spent. As for every XPS experiment one of the major limit of SPEMs was, indeed, the so called “pressure gap”, i.e. the highest pressure usable for in operando studies that was limited to high vacuum regimes. This problem was partially solved for photoemission spectroscopy setups at the beginning of this century but the found solutions could not be extended to the spatially resolved systems too. Here we presented an overview of the experimental capabilities of the SPEM hosted at Elettra where together with the classical UHV experiments it is now possible to perform in operando samples characterization at pressures as high as 1 mbar with a special setup called NAP-Cell. Two cases have been presented: (i) the investigation of Pt nanoparticles created on thin suspended, and supported, graphene substrates, which is a model system for a large class of 2D materials, and (ii) the oxidation of a Ru polycrystalline sample performed at NAP conditions. In both cases the typical SPEM performance, i.e. a beam spot of 130 nm and an overall energy resolution at RT of 0.2 eV is preserved. In the first case in combination with transmission electron microscopy the formation and distribution of Pt nanocrystals has been shown: in the suspended regions a high density of small particles coexists with regions with a lower density of agglomerated clusters formed by larger particles, while on the supported Gr only the small particles are present.
In the second experiment the oxidation was performed at 0.1 mbar; the inhomogeneous distribution of the amount of oxide phase on the flat surface areas was related to the distribution of the Ru grains and their orientation clearly indicating that the surface orientation has an effect on the oxide formation. The presented data prove the necessity of spatial resolved measurements to investigate and understand the oxidation process of Ruthenium.
The two examples show the flexibility of the SPEM in terms of measuring capabilities and sample environment.
We acknowledge Elettra Sincrotrone Trieste for provision of synchrotron radiation facilities and we would like to thank all the supporting services for assistance in using the beamline Escamicroscopy.
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