The aberration-corrected SPELEEM installed in our microscopy lab is a unique electron microscope capable of extracting structural, electronic, chemical, and elemental information with spatial resolution down to a few tens of nanometers. Equipped with a LaB6 electron cathode, it allows us to image elastically backscattered low-energy electrons from surfaces using low-energy electron microscopy (LEEM). Thanks to the high backscattering cross-section of low-energy electrons and the non-scanning imaging principle of the cathode immersion lens, we can achieve real-time imaging at video frame rates, making it ideal for monitoring in situ dynamic processes like nanostructure and thin-film growth and self-organization. With aberration correction, our LEEM offers lateral resolution better than 2 nm.
Simplified schematic of our SPELEEM instrument
Low energy electron microscopy (LEEM)
In LEEM, images form through various diffraction and phase contrast mechanisms. For bright-field imaging, we use the spectral diffracted beam, where local variations in sample structure generate diffraction contrast. Dark-field imaging, achieved using non-specular diffracted beams, is particularly effective for identifying coexisting crystal phases with different rotational variants.
Surface topology changes or potential variations induce local phase shifts in reflected electrons, generating phase contrast. For instance, atomic step height differences introduce a phase difference in backscattered electron waves, and the resulting interference provides contrast, allowing us to image atomic steps. Similarly, thin films or 2D layered materials of varying thickness show contrast due to interference between electron waves backscattered from the surface and the film/substrate interface. Depending on film thickness and electron energy, we observe intensity maxima and minima, enabling quantitative measurement of film thickness.
Low energy electron diffraction (LEED)
In addition to direct surface imaging, we can perform reciprocal space imaging for low-energy electron diffraction (LEED), a critical technique for studying crystalline surface structures. By inserting a small aperture in the electron beam path, we can restrict the electron beam footprint to under 250 nm, allowing micro-LEED (µ-LEED). This capability has enabled us to investigate structural deformation [1] and phase transitions [2] in atomically thin 2D materials.
[1] M. K. L. Man, et al., Scientific Reports 6, 20890 (2016).
[2] M. Rumo et al., Phys. Rev. B 101, 235120 (2020).
Bright and dark field images of CVD grown MoS2 sample
Photoemission electron microscopy (PEEM)
In addition to imaging sample surfaces using the electron cathode, our electron microscope also allows imaging with photoelectrons generated by UV or x-ray light sources through the photoelectric effect. These photo-emitted electrons carry information about the chemical and electronic environments from which they are emitted. Photoemission techniques provide insights into valence energy levels and the chemical bonding states of the near-surface region. Using the imaging energy analyzer, we achieve spatially resolved photoemission spectroscopy (PES/UPS/XPS) with lateral resolution better than 15 nm and energy resolution better than 150 meV. In recent years, we have applied PEEM to study hybrid halide perovskite photovoltaics, identifying various defect clusters and exploring their impact on charge trapping and performance losses [1–3].
[1] S. Macpherson et al., Nature 607, 294-300 (2022).
[2] S. Kosar et al., Enery Environ. Sci. 14, 6320-6328 (2021).
[3] T.A.S. Doherty et al., Nature 580, 360 (2020).
PEEM images showing the granular structure of solution processed perovskite
Besides looking at sample surfaces in real space, in the same microscope we can also do reciprocal-space imaging to resolve photoemitted electrons at different azimuthal and polar angles and energies. This allows for photoemission electron diffraction (PED) and angle-resolved photoemission spectroscopy (ARPES), facilitating studies of electronic structures and high-resolution 3D Fermi surface reconstruction.
Our lab has several UV light sources available for these studies. Surface imaging can be performed using a quasi-continuous wave UV laser or an ultrafast pulsed UV laser, while ARPES can be conducted with a He plasma discharge light source or an ultrafast pulsed extreme UV HHG light source.