Low Energy Electron Microscopy

Scanning Tunneling Microscopy (STM) allows imaging conductive surfaces at the atomic scale. It is possible to characterize the distribution of surface terraces and steps, as well as to determine the atomic arrangement of (ordered) surface (over)structures.

In STM, an atomically sharp tip is scanned on a surface at a few-angstrom distance, while a bias voltage is applied between these two electrodes, so that a current flows due to the quantum tunneling effect. The intensity of the tunneling current depends exponentially on the tip-surface distance and can therefore be used to reconstruct a morphologic image.

STM is a local technique: while high-resolution can be achieved on small (nanometer sized) areas, information on large-scale (micron sized or more) is lost, and measurements have to be repeated systematically on several regions of the sample to get statistically relevant information.

Due to stability performances, STM experiments are typically time-consuming. The technique is applicable both in air and in vacuum. Ultra-high-vacuum (UHV) is required for the characterization of delicate, atomically clean systems and for performing measurement at cryogenic temperature.

The STM signal is not purely topographic, but brings also information on the local density of electronic states. Scanning tunneling spectroscopy (STS) is an extension of STM that provides information about the density of electrons in a sample as a function of their energy. Inelastic tunneling spectroscopy (IETS) is a challenging extension for the investigation of vibrational states at liquid helium temperature. The STM tip can also be used to manipulate single atoms and molecules.

By acquiring sequences of consecutive images, STM can also be used to investigate at the atomic scale dynamical processes occurring on the surface of conductive samples, with a typical acquisition time of few tens of seconds per image. To further extend the range of accessible details in this kind of measurements, NFFA-Europe makes for the first time available to external users the access to a FastSTM option for high-speed imaging with a VT-STM microscope at CNR-IOM.  Thanks to this option, it is now possible to image with atomic resolution dynamical processes as chemical reactions, diffusion and growth, with a frame rate up to 100 images per second on regions few-nanometer wide.

SPELEEM microscope uses low-energy electron as information carriers. Unlike scanning microscopy, electrons are collected simultaneously from an illuminated area of several tens of micrometers. The image formed by the magnification lenses can be acquired even in video-rate. SPELEEM is capable of combining microscopy, spectroscopy and diffraction to obtain a comprehensive characterization of conductive and semiconductive samples from the meso- to the nanoscale. Thanks to its versatility, SPELEEM can be employed to study model systems in the fields of nanotechnology, material science, catalysis, energy storage, organic and inorganic thin films, and low-dimensional materials.

SPELEEM is equipped with an hemispherical energy analyzer and two complemetary illumination sources - an electron gun and a He I/II UV lamp. Using the UV lamp, the following operational modes are available.

• Photo-Emission Electron Microscopy (PEEM): the photoemitted electrons are collected by the optical elements to form a magnified image on a 2D electron detector. Electrons can be filtered in energy and acceptance angle while traveling through the optical path. The contrast mechanism is given by the physics behind the photoemission process and the chemical state of the probed matter.
• Micron-sized Angle-Resolved PhotoEmission Spectroscopy (µARPES): by displaying the backfocal plane of the objective lens on the 2D electron detector, the microscope give access to a high-quality reciprocal space map of the photoemitted electrons. The probed kinetic energy of the photoemitted electrons can be filtered and varied to collect a complete characterization of the occupied band structure of the specimen. The photoemitted electron beam can be segmented to a few-micron sized area, enabling the collection of ARPES maps from a controlled portion of the sample.

Using an electron gun, LEEM mode can be implemented in the same instrument.

SPELEEM experimental facility can accommodate conductive and semiconductive samples hosted by Elmitec and Flag-style sample holders. Heating stages are provided in the microscope chamber and in the preparation chamber. Other equipment available includes a gas line with precision leak valves, e-beam evaporator, preparation chamber with noble-gas sputtering, fast entry lock for sample loading.