Optical-luminescence

Atomic Layer Deposition (ALD) is similar to CVD except that the ALD reaction occurs at lower temperature and it is broken into two half-reactions, keeping the precursor materials separated during the reaction. The separation of the precursors is important to achieve a self-limiting surface reaction to enable precise thickness control and ensure uniform coatings and compactness even in complicated 3D structures.

The present implementation of the technique is suitable for mm to cm-size samples as those typically studied by spectroscopy and microscopy.

ALD has a rich history in microelectronics. It is studied as a potential technique to deposit high-k (high permittivity) gate oxides, high-k memory capacitor dielectrics, ferroelectrics, and metals and nitrides for electrodes and interconnects.

The number of materials that can be prepared by ALD has tremendously increased in the past decade. Notably, it can deposit a wider variety of materials, including important functional oxides like Al₂O₃, MoO₃, SiO₂ and HfO₂.

Ultrafast spectroscopy (or Pump-probe spectroscopy) is a technique used to study the dynamics of excited states in various materials, such as molecules, semiconductors, and solids.

It involves the use of two laser pulses. The first laser pulse, the "pump," excites the sample, elevating electrons from the ground state to an excited state. This initiation creates a non-equilibrium condition in the sample. After a controlled delay, a second laser pulse, the "probe," is sent through the sample. The probe pulse interrogates the sample to gather information about the transient states and the evolution of the excited states over time.

By varying the delay time between the pump and probe pulses, researchers can create a time-resolved picture of the dynamic processes occurring in the material. This time resolution can be on the order of femtoseconds (10^-15 seconds), allowing scientists to observe ultrafast phenomena.

These techniques provide insights into ultrafast processes, such as electronic transitions, high-order harmonics generation, molecular vibrations, and energy transfer mechanisms, that occur on very short time scales. It helps in understanding the fundamental properties of materials, such as the relaxation times of excited states, carrier dynamics in semiconductors, topological effects and the mechanisms of photochemical reactions.

Other possible studies can cover the world of solar cells, light-emitting devices, and photocatalysts, where understanding the dynamics of excited states is essential for improving efficiency and performance. Moreover, in biophysics and biochemistry, pump-probe spectroscopy can be used to investigate the dynamics of biomolecules, such as the folding and unfolding of proteins, photosynthesis mechanisms, and the behavior of molecular motors.

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.

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.