Photoemission Electron Microscopy Study Research Articles

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.

Present status and recent progress of research, using photoemission-electron microscopy at SPring-8

Photoemission electron microscopy (PEEM), particularly when combined with synchrotron radiation, exerts a powerful functionality in spectroscopy applications. The use of PEEM has steadily expanded the availed analysis objects at the SPring-8 synchrotron facility, such as magnetic materials, thin-film devices, semiconductors, monolayers, extraterrestrial matter, and multiferroics. Some special experimental settings such as time-resolved or operando measurements have also been developed according to user requests. In this paper, we present an overview on the achievements of PEEM studies so far and introduce some recent technical and scientific progress at SPring-8.

A near ambient pressure photoemission electron microscope (NAP-PEEM)

Photoemission electron microscopy (PEEM) is a powerful surface technique for dynamic imaging of surface processes while PEEM studies have been performed under ultrahigh vacuum or high vacuum conditions. Here we report on a near ambient pressure PEEM (NAP-PEEM) instrument, which enables high resolution PEEM imaging in near ambient pressure (> 1 mbar) gases over a wide temperature range (150 – 1200 K). Installed with an electron gun near ambient pressure low energy electron microscopy (NAP-LEEM) can be achieved as well. The success of this new NAP-PEEM/LEEM instrument relies on the following key design concepts. First, a two-stage accelerating electric field consisting of a low field region between sample and intermediate electrode (“nozzle”) and a high field between nozzle and objective lens. Second, a three-stage differential pumping system allowing a near ambient pressure atmosphere at the sample surface while ultrahigh vacuum maintained in the imaging lens systems. Third, a unique NAP cell with gas inlet/outlet, light illumination, sample cooling/heating, and precise sample positioning. The new technique will have important applications in surface catalysis, thin film growth, and energy conversion devices under nearly realistic working conditions.

Photoemission Electron Microscopy as a Tool for Studying Steel Grains

Key properties of steel like stability, weldability, or ability for absorbing deformation energy are defined by their grain structure. The knowledge about their micrometer and submicrometer structure is of particular interest for tailor-cut macroscopic steel properties. We report on photoemission electron microscopy studies which in principle yield a higher magnification than comparable optical techniques. A flat surface without any topographic features was obtained by applying a non-etching preparation procedure. PEEM images showed very tiny phase islands embedded within a steel phase matrix. Furthermore, we developed an analysis procedure for PEEM images for dual-phase steels. As a result, it is possible to identify the individual work functions of different steel phases at the surface.

Inner photoelectric effect at Pt/YSZ interface during photoemission electron microscopy

We demonstrate in a photoemission electron microscopy (PEEM) study of Pt/YSZ in vacuum that an open circuit (OC) potential can develop which we attribute to an inner photoelectric effect caused by the irradiation with UV light. The appearance of an OC potential is linked to the existence of a high voltage (HV) of at least 5kV between sample and PEEM instrument. The OC potential grows with increasing HV reaching about −200mV at a HV of 12kV. The effect represents a potential artefact in PEEM studies of solid ionic conductors with sufficiently small band gap (<6eV for photons from D2 discharge lamp).

Nanoscale sub-100 picosecond all-optical magnetization switching in GdFeCo microstructures.

Ultrafast magnetization reversal driven by femtosecond laser pulses has been shown to be a promising way to write information. Seeking to improve the recording density has raised intriguing fundamental questions about the feasibility of combining ultrafast temporal resolution with sub-wavelength spatial resolution for magnetic recording. Here we report on the experimental demonstration of nanoscale sub-100 ps all-optical magnetization switching, providing a path to sub-wavelength magnetic recording. Using computational methods, we reveal the feasibility of nanoscale magnetic switching even for an unfocused laser pulse. This effect is achieved by structuring the sample such that the laser pulse, via both refraction and interference, focuses onto a localized region of the structure, the position of which can be controlled by the structural design. Time-resolved photo-emission electron microscopy studies reveal that nanoscale magnetic switching employing such focusing can be pushed to the sub-100 ps regime.

Interplay between intrinsic and stacking-fault magnetic domains in bi-layered manganites

We present a low temperature x-ray photoemission electron microscopy study of the bi-layered manganite compound La1.2Sr1.8Mn2O7 (BL-LSMO) to investigate the influence of stacking faults, which are structurally and magnetically different from the bi-layered host. In BL-LSMO, small magnetic moment persists to T*=300 K, well above the Curie temperature of 120 K (TC). Our magnetic images show that 3D stacking faults are responsible for the T* transition. Furthermore, close to the TC, stacking faults are well coupled to the bi-layered host with latter magnetic domains controlling the spin direction of the stacking faults. Contrary to recent reports, we find that stacking faults do not seed magnetic domains in the host via an exchange spring mechanism and the intrinsic TC of the BL-LSMO is not lower than 120 K.

A low-energy electron microscopy and x-ray photo-emission electron microscopy study of Li intercalated into graphene on SiC(0001)

The effects induced by the deposition of Li on 1 and 0 ML graphene grown on SiC(0001) and after subsequent heating were investigated using low-energy electron microscopy (LEEM) and x-ray photo-emission electron microscopy (XPEEM). For 1 ML samples, the collected photoelectron angular distribution patterns showed the presence of single π-cones at the six equivalent K-points in the Brillouin zone before Li deposition but the presence of two π-cones (π-bands) after Li deposition and after heating to a few hundred °C. For 0 ML samples, no π-band could be detected close to the Fermi level before deposition, but distinct π-cones at the K-points were clearly resolved after Li deposition and after heating. Thus Li intercalation was revealed in both cases, transforming the carbon buffer layer (0 ML) to graphene. On 1 ML samples, but not on 0 ML, a (√3×√3) R30° diffraction pattern was observed immediately after Li deposition. This pattern vanished upon heating and then wrinkles/cracks appeared on the surface. Intercalation of Li was thus found to deteriorate the quality of the graphene layer, especially for 1 ML samples. These wrinkles/cracks did not disappear even after heating at temperatures ⩾500 °C, when no Li atoms remained on the substrate.

Nanoscale analysis of Ru(0001) oxidation using low-energy and photoemission electronmicroscopy

CO oxidation over oxygen-rich Ru(0001) surfaces is one of the most studied catalyticoxidation reactions in surface science and of widespread interest as a model system for theredox chemistry of transition metal model catalysts. Here, we present an extensivelow-energy electron microscopy (LEEM) and photoemission electron microscopy study ofthe oxidation of Ru(0001), which constitutes a crucial step in understanding the overallsurface reaction. After characterizing the different surface nanoscale morphologiesobserved depending on the oxidation temperature, three distinct oxygen-richphases are identified by dark-field microscopy and local valence-band spectroscopy.Furthermore, in situ LEEM allows us to follow the growth of single rutile oxidenuclei in real time and determine the relevant activation barriers that inducequasi-one-dimensional growth of oxide nanorods, whose growth rate is limited byO incorporation.

An x-ray photoemission electron microscopy study of the formation of Ti–Al phases in 4 mol%TiCl3 catalyzedNaAlH4 during high energy ball milling

This study reports reaction pathways to formTiAlx metallic complexes during the high energy ball milling of 4 mol%TiCl3 withNaAlH4 powders determined using local structure analysis ofTix+ andAlx+ species.Using x-ray photoemission electron microscopy (XPEEM) and x-ray diffraction (XRD), the oxidationstate of Alx+ and Tix+ and the crystalline compounds existing in equilibrium withNaAlH4 were tracked for samples milled for times of 0 (i.e. mixing), 5, and 25 min. XPEEManalysis of the Al K edge after 5 min of milling reveals that Al remains in the3+ oxidationstate (i.e. in NaAlH4) around Ti0-rich regions of the sample. After 25 min of high energy milling,Ti0 has reactedwith Al3+ (innearby NaAlH4)to form TiAlx complexes. This study reports the pathway forTiAlx complex formationduring milling of 4 mol% TiCl3 catalyzed NaAlH4 to beas follows: (1) Ti3+ reduces to Ti0 (with Al3+ nearTi0 regions)and (2) Ti0 reacts with Al3+ in NaAlH4 toform TiAlx complexes.

Microscopic investigation of the CdS buffer layer growth on Cu(In,Ga)Se2 absorbers

To obtain highly efficient thin film solar cell devices, a thin buffer layer of CdS is deposited onto a Cu(In,Ga)Se2 absorber layer. Here, the authors report a photoemission electron microscopy study investigating the growth of evaporated CdS buffer layers. For thin CdS layers (&amp;lt;1nm thickness), they observed enhanced CdS growth on some grains, while other grains remain uncovered or only weakly covered. For thicker CdS layers, they observed a more homogeneous growth. After annealing these layers, an increase in the detailed In and Se structure is observed as well as the reappearance of the inhomogeneous Cd distribution. This indicates an intermixing of CdS and Cu(In,Ga)Se2. The influence of such interdiffusion on solar cell efficiency is discussed.

Atomic force microscopy and photoemission electron microscopy study of the low-pressure oxidation of transition metal nitrides

The oxidation of transition metal nitrides is examined by atomic force microscopy (AFM) and photoemission electron microscopy (PEEM) in the temperature range of 25–800°C in an ultrahigh vacuum study. The roughness of the surfaces of thin film coatings of vanadium (V), vanadium nitride (VN), titanium nitride (TiN), and TiAlVN, monitored as a function of (a) constant dose of 50000L (1L=10−6Torrs) O2 at various temperatures and (b) oxygen dose at a constant temperature of 800°C, is found to increase for all samples except TiAlVN. The morphological parameters (roughness and oxide grain volume) obtained from AFM are used to determine the extent of oxidation. The activation energy for the growth of oxide on the surface is calculated to be in the range of 0.4–0.5eV for all samples containing V atoms and 1.6eV for TiN. PEEM images of the pristine surfaces of TiN and TiAlVN show surface precipitates of the order of 5–50μm size. The workfunction of all surfaces and precipitates, obtained from photoemission spectra using PEEM, in combination with the effective activation energies measured by AFM, suggests that the main surface precipitate of TiAlVN is VN or VOxNy.

Photoemission electron microscopy study of anthracene growth on Si(1 1 1)

The thin film growth of anthracene films on Si(1 1 1) surfaces is studied by photoemission electron microscopy (PEEM). The thin film growth of anthracene on Si(1 1 1) is similar to the growth of pentacene on silicon. Initially a layer of flat lying molecules chemisorbs on the surface. Subsequent growth of fractal islands with standing up molecules proceeds on top of this flat layer.

Photoemission electron microscopy study of remanent magnetic domain states in ferromagnetic wedge films deposited on substrates with micrometer-sized square plateaus

We present a detailed study of the remanent magnetic domain configurations in demagnetized polycrystalline ferromagnetic thin film wedges of cobalt and Permalloy deposited on prepatterned silicon substrates with micrometer-sized square plateaus, which have a height of 125 nm, using photoemission electron microscopy. We have observed the continuous evolution of the magnetic domain states in the square ferromagnetic elements on top of the plateaus as a function of film thickness. At high film thickness we observe the Landau state, which is the expected lowest energy state, but at lower thickness we see a variety of metastable states which are trapped as a result of local pinning. In a small thickness range below 10 nm, the square elements contain 360° walls and small domains which are likely to be a result of local effects such as magnetocrystalline anisotropy and edge roughness. We are able to simultaneously observe the development of the magnetic domains in the continuous polycrystalline film surrounding the plateaus and, rather than the expected large domains, we observe at intermediate film thickness a significant modification of the domain configuration to small domains. Here the roughness of the silicon substrate surrounding the plateaus, which is due to the reactive ion etching process used to prepare the prepatterned substrates, gives rise to local stray fields in the ferromagnetic film which play an important role in determining the resulting domain structure.

Selective growth of Co nanoislands on an oxygen-patterned Ru(0001) surface

We present a combined low energy electron microscopy and photoemission electron microscopy study of Co nanoislands grown on pure and partially oxidized Ru(0001) surfaces at 650--850 K. When Co is deposited on pure Ru(0001) at the temperature above $\ensuremath{\approx}700\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, it first forms a wetting monolayer and then Co nanoislands abruptly nucleate. Partial oxidization of the clean Ru(0001) surface gives rise to large rhombus-shaped Ru--O patches (of $10\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ scale). On this surface at $\ensuremath{\approx}730\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the Co islands form with different density in the Ru and Ru--O regions, and at $\ensuremath{\approx}800\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the Co islands only grow in the pure Ru regions. We attribute this selectivity to differences in the temperature dependence of the sticking coefficients for Co on the pure Ru and Ru--O regions.

Solid phase reaction in Ti(thin film)/Si(substrate) with Mo interlayer: SXES and PEEM study

Soft X-ray emission spectroscopy (SXES) and photoemission electron microscopy (PEEM) were performed to study the solid phase reaction in the Ti(film)/Si(substrate) system with Mo interlayer. SXES is a suitable technique for non-destructive buried-layer characterization and is capable of qualitative analysis and chemical bonding state determination. PEEM is an imaging system for real-time observation of surface structures. The Si L 2,3 soft X-ray emission spectra of a Ti(50 nm)/Mo(12 nm)/Si(100) sample, annealed at 550 °C for 5 min, revealed formations of majority mixed species of C54-TiSi 2 and MoSi 2. Using PEEM imaging, the surface of a Ti(7.5 nm)/Mo(2.5 nm) bilayer film on Si(111) became rough due to the formation of some island-like structures after annealing to 500 °C.

PEEM, AES and SXES Studies of Cu(Film)/3C-SiC

PEEM (photoemission electron microscopy), AES (Auger electron spectroscopy) and SXES (soft X-ray emission spectroscopy) studies have been carried out on Cu(thin film)/3C-SiC(substrate) contact systems. The PEEM studies have clarified both lateral diffusion and microstructure formation on the Cu/3C-SiC specimen surfaces due to heat treatment. The AES has shown accumulation of carbon atoms on top of the specimen surface at high temperature annealing. The SXES studies have elucidated a silicide, i.e., Cu3Si, formation for the heat treated Cu(film)/3C-SiC specimen. A structural model is proposed for the heat treated Cu(30 nm)/3C-SiC(substrate) specimen. [DOI: 10.1380/ejssnt.2004.230]

Surface and interface of Ti(film)/SiC(substrate) system: a soft X-ray emission and photoemission electron microscopy study

We have conducted a soft X-ray emission spectroscopy (SXES) and a photoemission electron microscopy (PEEM) study on the heat-treated Ti/4H–SiC system. This spectro-microscopy approach is an ideal surface and interface characterization techniques due to the non-destructive nature of SXES and the real-time surface imaging of PEEM. The Si L 2,3 and C K soft X-ray emission spectra, which reflect Si (s+d) states and C p states, respectively, revealed formations of Ti 5Si 3 and TiC in the reacted interfacial region of Ti (50 nm)/4H–SiC(0 0 0 1) sample. The surface of the Ti films on 4H–SiC samples during heat-treatment up to ∼850 °C was investigated by PEEM. The variation in brightness in the image of the sample was attributed to the surface deoxidation in the early stage of the treatment and to the formation of reacted region at the later stage. The darkening of the surface could be attributed to the formation of TiC and/or excess C atoms that could have migrated to the surface.

Investigation of the NO + H2 reaction on Pt{100} with low-energy emission and reflection microscopy: PEEM studies

Using the technique of photoemission electron microscopy (PEEM) we have examined the reaction between NO and hydrogen on Pt{100} during transient changes in the partial pressures of the reactants. On switching to a hydrogen-rich ambient, reaction–diffusion fronts are formed, the nucleation, velocity and shape of which vary strongly with temperature and the rate of change in partial pressure. Behind the fronts an adsorbate, probably NH3, produces low workfunction areas on the surface. Below 370 K, however, a different adsorbate, possibly HNO, is formed.