Ultrafast-Spectroscopy

AFM is a surface sensitive technique permitting to obtain a microscopic image of the topography of a material surface. Typical lateral image sizes are within a range of only a few Nanometers to several 10 Micrometers, whereas height changes of less than a Nanometer may be resolved.

A fine tip attached to a cantilever is scanned across the material surface and enables to measure height changes exploiting an optical beam deflection system (a laser that is reflected from the rear side of the cantilever onto a segmented photodiode). The position of a laser spot on the photodiode permits to track height changes as e.g. due to a nano-particle on the surface or an atomic terrace of a single crystal surface. A feedback loop controls the tip-surface distance and therefore ensures stable imaging conditions.

Different operation modes like contact or non-contact mode can be used to optimize the imaging conditions with highest lateral resolution on one hand and least sample interaction on the other hand. Measurements in different environmental conditions (liquid, gas,...) on a broad class of samples (smooth, rough, insulating, conductive, soft, stiff, wet, dry...) can be performed.

Choosing suitable tips and imaging modes, additional surface properties can be mapped together with the topography, with similar spatial resolution, like friction force, magnetization and surface potential, surface charge density and electrical resistance, as well as elastic modulus and adhesion of heterogeneous sample surfaces can be obtained. 

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.

Transmission Electron Microscopes (TEM) and Scanning Transmission Electron Microscopes (STEM)  are designed for high-resolution imaging and analysis.
In both cases, high energy electrons, incident on ultra-thin samples called TEM lamella (typically <100nm), travel through the specimen.
Depending on the density, crystallinity, orientation, etc. of the material present, electrons are scattered differently, giving rise to an image of the sample with different contrast features according to specimen properties and to the microscope setup.
Usually, the magnified image of the sample is focused on a CCD or CMOS camera. On the same system is also possible to acquire the electron diffraction produced by a specific area of the sample.

Modern TEM are equipped with a Cold Field Emission Gun (FEG) or with a Schottky FEG, making them suitable for materials science, nanotechnology, and various fields requiring detailed structural and compositional analysis in a local nanoscale. Indeed, point resolution below 0.3 nm at 200kV in TEM mode can be reached, according to the
instrument configuration.

In the STEM mode, electrons pass through the specimen, but, the electron optics focus the beam into a narrow spot which is scanned over the sample in a raster. The rastering of the beam across the sample makes these microscopes suitable for analysis techniques.

According to the specific configuration (see details and the specific availability below, referred to each instrument and facility), the microscope can be equipped with a selection of multiple detectors enabling different techniques, such as Energy-Dispersive x-ray Spectroscopy analysis (EDS), electron energy loss spectroscopy (EELS), STEM detectors in bright field (BF), annular bright field (ABF), annular dark field (ADF) for diffraction contrast, high-angle annular dark field (HAADF) for Z-contrast.

EELS/EDS system control software enables point-by-point signal acquisitions, for the analysis of spectra and maps.
Sample preparation (see details and the specific availability below, referred to each instrument and facility) is a crucial part in TEM experiments.

High quality TEM specimens have a thickness that is comparable to the mean free path of the electrons that travel through the samples, which may be only a few tens of nanometres. Preparation of TEM specimens is specific to the material under analysis and the desired information to obtain from the specimen. Sample preparation laboratories are equipped with the basic tools (diamond saw, polisher, dimpler, ultrasonic cutter, precision ion polishing system, plasma cleaner) commonly used in conventional thinning procedures.

Luminescence optical spectroscopy is a classical spectroscopy to characterize excited states in semiconductors. It is based on an optical excitation with an excess energy with respect to the band gap. The various pathways of de-excitation give rise to a re-emitted photon distribution with energy related to the excited states.

The lineshape as a function of the re-emitted photon energy is generally modeled by the product of the absorption times the photoexcited electron and hole distribution functions at energies coupled by the photon. In this approximation a common temperature of electrons and holes is postulated and by fitting the high energy tail of the spectrum the carriers temperature can be retrieved.

The PhotoLumiscence also provides a scenario for the exciton dynamics; in particular, varying the flux of the photon beam, the PhotoLuminescence intensity is super-linear in the case of exciton-exciton interaction.

Moreover, PhotoLuminescence at low temperature is a powerful tool to characterize point defects in semiconductors, and it is a fundamental characterization to improve the quality of the semiconductors in electronic devices.

PES (Photo-electron Spectroscopy) is a general term that includes X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA),  and Ultraviolet Photoelectron Spectroscopy (UPS), that are the most used techniques for the investigation of the chemical properties of the material. In PES, a monochromatic photon beam impinges on the sample and extracts photoelectrons. These photoelectrons are then collected by an electrostatic lens system; their energy dispersion is then measured by an electron energy analyzer. The collected data form the so-called electron dispersion curve (EDC) or photoemission spectrum. From the analysis of the photoemission spectrum, it is possible to derive the chemical composition (stoichiometry) of the material under study and several information about the chemical state of each element, such as oxidation state, chemical bonds, etc. PES can be exploited as a surface chemical analysis technique: the valence band DOS (assessable by UPS) reveals the orbital contributions through cross-section analysis (photon energy dependence) and core level photoemission (assessable by XPS) is sensitive to the local environment of a given atomic species in the first atomic layers of the sample.