Cathode lens spectromicroscopy: methodology and applications

Abstract

The implementation of imaging techniques with low-energy electrons at synchrotron laboratories allowed for significant advancement in the field of spectromicroscopy. The spectroscopic photoemission and low energy electron microscope, SPELEEM, is a notable example. We summarize the multitechnique capabilities of the SPELEEM instrument, reporting on the instrumental aspects and the latest developments on the technical side. We briefly review applications, which are grouped into two main scientific fields. The first one covers different aspects of graphene physics. In particular, we highlight the recent work on graphene/Ir(100). Here, SPELEEM was employed to monitor the changes in the electronic structure that occur for different film morphologies and during the intercalation of Au. The Au monolayer, which creeps under graphene from the film edges, efficiently decouples the graphene from the substrate lowering the Dirac energy from 0.42 eV to 0.1 eV. The second field combines magnetism studies at the mesoscopic length scale with self-organized systems featuring ordered nanostructures. This example highlights the possibility to monitor growth processes in real time and combine chemical characterization with X-ray magnetic circular dichroism-photoemission electron microscopy (XMCD-PEEM) magnetic imaging by using the variable photon polarization and energy available at the synchrotron source.

Keywords: X-ray magnetic circular dichroism (XMCD); X-ray photoemission electron microscopy (XPEEM); gold (Au); graphene; intercalation; low-energy electron microscopy (LEEM); magnetism; nanostructures.

Figure 1

The simplified schematic description of a) XPEEM, b) LEEM. The energy analyzer (EA) is optional in both cases. Panel (b) with the energy analyzer represents also the SPELEEM setup.

Figure 2

Energy dependence of the (00) beam intensity for clean, Fe-covered and O-covered W(110) surfaces. The top panels show the respective LEED patterns. The inset is a blowup of the MEM–LEEM transition at low energy. The increase (decrease) in the work function due to the presence of O or Fe is seen in the shift of the transition energy.

Figure 3

a) Illustration of imaging spectroscopy in XAS mode. Fe nanowires on W(110) appear dark on the left panel at a photon energy of 704.5 eV. At the Fe L₃ threshold, the wires become much brighter (middle panel). The XAS spectrum below is extracted from the largest nanowire in the center. b) Illustration of XMCD-PEEM imaging. The photon energy is tuned to the L₃ maximum. The field of view is 5 μm. The start voltage is 3 eV in order to collect secondary electrons. Within the image plane, the X-ray direction is perpendicular to the nanowire axis.

Figure 4

The SPELEEM instrument at the Nanospectroscopy beamline, Elettra Sincrotrone, Trieste. The sketch of the basic setup is superimposed onto the photograph. X-rays arrive from the right at 16° grazing angle to the sample surface.

Figure 5

a) The energy distribution of the electron beam emitted from the LaB₆ source acquired by keeping the sample below the MEM transition using a negative start voltage bias. The intensity-vs-energy curve is obtained in dispersive plane operation, in which the exit plane of the energy analyzer is projected onto the detector. b) The (00) LEED spot profile from W(110).

Figure 6

Tungsten 4f_7/2 core level spectrum from a clean W(110) surface acquired in dispersive plane mode. The photon energy is 90 eV. The acquisition time is 80 s. The Lorentzian broadening for the bulk peak was fixed at 60 meV, with an asymmetry parameter of 0.035. The contrast aperture, which acts as the analyzer entrance slit, is 20 μm.

Figure 7

Lateral resolution in LEEM. The inset shows a Ni monolayer island (dark) on W(110). The profile in the plot is marked on the image. The full width of the sigmoid function is 8.2 nm. Averaging over several profiles, the value is found to be 8.6 ± 1.2 nm.

Figure 8

LEEM images at a start voltage of 12 eV illustrating the evolution of the graphene/Ir(100) interface upon deposition of Au. A large graphene crystal (gr) is visible in all images (lower half), brighter than the Ir surrounding it (upper half). (a) Initial configuration of the sample at T = 520 °C. (b) The same area after a dose of 0.25 ML Au at sample temperature of 600 °C. Au (dark areas) has decorated steps and step bunches. (c) The same area after a dose of 0.85 ML of Au. At this stage, Au has entirely covered the initially bare Ir surface and the intercalation under graphene has just started (darker areas). Note also that small graphene islands have nucleated on the Au/Ir surface. (d) The same area after a dose of 0.9 ML of Au.

Figure 9

Graphene on Au/Ir(100) (left column) and Ir(100) (right column). (a) μ-ARPES near E_F; the high symmetry points in the first Brillouin zone (FBZ) are indicated. Photon energy is 40 eV. The probed area has a diameter of 2 μm. (b) Momentum distribution curves along the normal to the Γ–K direction, as indicated by the dashed line in (a).

Figure 10

a) LEEM images (2 μm diameter) of monolayer Pd stripes on W(110). The lower left panel shows Pd on a clean substrate, whereas the other two panels display the progressive change in pattern anisotropy upon addition of 0.1 ML and 0.33 ML oxygen. (Reprinted from [69]. Copyright 2011 IOP Publishing.) b) Fe grown on Pd–O stripes at 225 °C. Left panel is the XAS-PEEM image at the Fe L₃ edge showing the Fe distribution. On the right, the Fe XMCD image indicates that the wires are uniformly magnetized perpendicular to the long axis. (Reprinted from [70]. Copyright 2013 Elsevier.)

Figure 11

a) Magnetite islands and the FeO wetting layer on Ru(0001). Top panels show the island and magnetization distribution within a region of 30 μm diameter, illuminated homogeneously by vertically scanning the photon beam during acquisition. Bottom panels show the details of the magnetic domains (left, field of view 4 μm) and the Fe L₃ XMCD spectrum extracted from a single domain (right). (Reprinted with permission from [72]. Copyright 2012 American Physical Society.) b) Morphology of Fe₃O₄/Pt(111) from LEEM and μ-LEED spot profile analysis. The large tail in the (00) spot profile (seen on the left) is identified with the formation of oxygen-related agglomorates as sketched below. Middle panels show the effect of cooling/annealing on the spot profile. LEEM images on the right (at an energy of 24 eV) present the surface before and after the annealing cycle. (Reprinted with permission from [73]. Copyright 2012 American Physical Society.)