Structural and Surface Characterization
Contact person: Rossi Francesca
Photoelectron Spectroscopies
   CONTACT: Roberto Verucchi (roberto.verucchi@imem.cnr.it, tel 0461-314825) ; Letizia Savio (letizia.savio@imem.cnr.it, tel 010-3536292)

 

X-Ray photoelectron spectroscopy (XPS)

Setup

XPS setup in Genova

EA125 Omicron analyser @ IMEM-Genova

X-Ray source, DAR400 Omicron (Mg/Al anode, non monochromatic)
Installed on a UHV chamber with HREELS spectrometer and connectable to a supersonic molecular beam.  

XPS setup in Trento

VSW HA100 analyser with control unit PSP @ IMEM-Trento

X-Ray source, VG 8600 (Mg/Al anode, non monochromatic). Ion gun VG EX05. UHV system, connected to a co-deposition chamber and to a supersonic beam source enabling in situ analysis of material growth processes. 

 

 

 

Method

This spectroscopy exploits the photoelectric effect for the analysis of the surface composition. In fact, the electrons of the core levels are bound to the nucleus with a binding energy (Eb) characteristic of the element and of the occupied orbital. Therefore they are photo-emitted by the sample with a residual kinetic energy that makes them identifiable.  

XPS is a surface-sensitive analytical technique. It can be applied to conductive and semiconducting samples and, using a neutralizer, even to insulators. It provides quantitative information on:

  • Chemical composition of the surface
  • Chemical state of atoms and molecules at the surface
scheme

 

Highlights

Identification and quantitative analysis of chemical species on organic or inorganic surfaces

XPS on graphene
N-doped graphene on Ni(111). The different C species (corresponding to different graphene domains) and N species (dopant) are identified and quantified. Refs: Appl. Surf. Sci. 2018; J. Chem. Phys. 2017 

 

XPS on Pt
Decomposition of carbonyl Pt clusters on a defected graphene surface. Study of the Pt chemical state before and after thermal treatment. Ref: Surf. Sci. 2020

 

XPS on organic compounds
Piezoelectric quartz crystal functionalised with cavitands for the detection of alogenated aromatic hydrocarbons in water. Study of the chemical properties of C in the organic molecule. Ref: Sens. Act. B 2018

 

 

Ultraviolet Photoelectron Spectroscopy (UPS)

Setup

setup UPS

Electron energy analyser VSW HA100 with control unit PSP @ IMEM-Trento

UV source: custom made He discharge lamp, non monochromatized
Photon energy: He I (21.22 eV), He II (40.81 eV)
Working discharge current: 25-100 mA
Spot Ø ~1 mm
Sample bias: -7 V 

 

Method

UPS spectroscopy is based on photoelectric effect to analyze surface elctronic properties. The sampled electron energy levels are the most external ones, such as valence band for inorganics. Main informations that can be acquired are:
•    Fermi level identification 
•    Material work function
•    Ionization potential
•    Distribution of valence band (molecular orbitals for organics) states 
•    Charge transfer processes. 
When used together with XPS technique, it is possible to study formation of surface dipoles, band bending, charge accumulation.
It can be applied to conducting materials, as well as semiconductors (also with wide band gap), inorganics and organics. It can’t be used for highly insulating materials, or when degradation due to UV photon occurs. UPS requires sample negative polarization to enable electron energy analyzer proper working.

 

Highlights

spectra

T6/PDI-8CN2 heterostructures on Au substrate: presence of a significant charge transfer from the T6 HOMO to the perylene LUMO. An accumulation layer of 5–6 nm in T6 layer can be estimated. Valence band evolution (right) and energy levels alignments (left) are shown. Refs: Appl. Phys. Lett. 2012; Appl. Phys. Lett. (2014) 

spectra
Synthesis of single layer graphene on Cu(111) by C60 supersonic molecular beam epitaxy: valence band from clean metal surface (a), (b) 1ML C60 film deposited at RT (KE = 35 eV), after thermal annealing at 425°C (c), 645°C (d), 795°C (e) and from a commercial graphene single-layer on Cu foil (e).
Ref: RSC Adv. (2016) 
spectra
Valence bands of several metal oxides (energy scale is in kinetic and not binding energy. 
Ref: Opt. Mat. (2017) 
spectra
Valence bands of 2H-MoS2 single crystal after several thermal treatments, with formation of band bending and a surface dipole. 
Ref: J. Phys. Chem. C (2020)