XPS Techniques at MXP

Yvonne Hora & Junlin Yan

Monash X-ray Platform, Monash University

​

14^th November 2023

XPS Techniques at MXP

Monash X-ray Platform

OUTLINE

• X-ray Photoelectron Spectroscopy (XPS)
• XPS Techniques
• Sample Modules and Stages
• XPS of Air-Sensitive Samples

​

• Ultra-violet Photoelectron Spectroscopy (UPS)
• Reflected Electron Energy Loss Spectroscopy (REELS)

​

X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) is a surface analysis technique

​

Identifies elements across to top 5 – 10 nm of surfaces

​

XPS requires ultra-high vacuum (<10^-9 mbar) conditions to measure

• Elemental composition
• Chemical states
• Electronic states of the elements

​

XPS spectra are obtained by irradiating a material with a focused beam of X-rays

to measure the number and kinetic energy of the ejected electrons from the sample surface.

​

X-ray Photoelectron Spectroscopy

- A material is irradiated with a focused beam of X-rays

​

- Measures the kinetic energy and number of the ejected electrons

​

- XPS spectra is generated

​

​

​

X-ray Photoelectron Spectroscopy

Thermo Scientific

Nexsa Surface Analysis System

Essential Instrument Hardware �Front View

Analysis Chamber

7

Flood Gun

Load Lock

FRONT

UV Source

Essential Instrument Hardware �Side View

8

Ion Gun

X-ray Gun

SIDE

UV Source

XPS Techniques

Instrument Sources

• X-RAY GUN
• ION GUN
• FLOOD GUN
• UV SOURCE

​

​

INSTRUMENT FRONT

Instrument Sources: X-ray Gun

• X-ray Photoelectron Spectroscopy (XPS)
• Micro-focused and Monochromated Al K-α X-ray source
• Quantified Chemical State Analysis
• Adjustable Spot Size (10 - 400 µm)
• Surface Information (5 – 10 nm)

​

​

​

INSTRUMENT FRONT

Instrument Sources: X-ray Gun

• X-ray Photoelectron Spectroscopy (XPS)
• Micro-focused and Monochromated Al K-α X-ray source
• Quantified Chemical State Analysis
• Adjustable Spot Size (10 - 400 µm)
• Surface Information (5 – 10 nm)

​

• X-ray Mapping (SnapMap)
• Elemental Mapping of a Sample Surface
• Measures the distribution of Chemical States
• Features as small as 10 µm can be identified
• Mapped Area (0.5 mm² – 3 mm²)

​

​

​

INSTRUMENT FRONT

Instrument Sources: X-ray Gun

• X-ray Photoelectron Spectroscopy (XPS)
• Micro-focused and Monochromated Al K-α X-ray source
• Quantified Chemical State Analysis
• Adjustable Spot Size (10 - 400 µm)
• Surface Information (5 – 10 nm)

​

• X-ray Mapping (SnapMap)
• Elemental Mapping of a Sample Surface
• Measures the distribution of Chemical States
• Features as small as 10 µm can be identified
• Mapped Area (0.5 mm² – 3 mm²)

​

• Angle-Resolved XPS (ARXPS)
• Tilt Module
• 0° - 89°

​

​

INSTRUMENT FRONT

Instrument Sources: Ion Gun

• Monatomic and Gas Cluster Ion Source (MAGCIS)
• Dual Mode Ion Source
• Monatomic Mode
• Hard Surfaces
• 500 eV – 4 keV ion energy
• Gas Cluster Mode
• Soft / Organic Surfaces
• 2 keV – 8 keV ion energy

​

INSTRUMENT FRONT

Instrument Sources: Ion Gun

• Monatomic and Gas Cluster Ion Source (MAGCIS)
• Dual Mode Ion Source
• Monatomic Mode
• Hard Surfaces
• 500 eV – 4 keV ion energy
• Gas Cluster Mode
• Soft / Organic Surfaces
• 2 keV – 8 keV ion energy

​

• Ion Beam Cleaning (Etching)

​

INSTRUMENT FRONT

Instrument Sources: Ion Gun

• Monatomic and Gas Cluster Ion Source (MAGCIS)
• Dual Mode Ion Source
• Monatomic Mode
• Hard Surfaces
• 500 eV – 4 keV ion energy
• Gas Cluster Mode
• Soft / Organic Surfaces
• 2 keV – 8 keV ion energy

​

• Ion Beam Cleaning (Etching)

​

• Depth Analysis
• Generates Depth Profiles using Atomic % Concentrations

INSTRUMENT FRONT

Instrument Sources: Flood Gun

• Charge Neutralisation of Insulating Surface
• Prevents Sample Charging
• Dual Beam Electron / Ion Source
• Low Energy Electrons
• Ar⁺ Ions

INSTRUMENT FRONT

Instrument Sources: Flood Gun

• Charge Neutralisation of Insulating Surface
• Prevents Sample Charging
• Dual Beam Electron / Ion Source
• Low Energy Electrons
• Ar⁺ Ions

​

• Reflected Electron Energy Loss Spectroscopy (REELS) - Junlin
• High Energy Flood Gun Mode
• Provides Electronic Structure Information
• Band Gap
• Can measure the presence of Hydrogen

INSTRUMENT FRONT

Instrument Sources: UV Source

• UV Photoelectron Spectroscopy (UPS) - Junlin
• Valance Band
• Work-function
• Sample Bias Module

​

INSTRUMENT FRONT

Sample Modules and Stages

Sample Modules and Stages

• Standard Stage
• Bias Sample Holder
• Rotation Holder
• Fibre Holder
• Tilt Sample Holder (Angle-Resolved XPS)

​

Vacuum Transfer Module

Transfer of air-sensitive samples �directly from a glovebox to the Nexsa

​

​

​

​

​

​

• Module is vacuum sealed within the glovebox antechamber
• Transfer to the Nexsa Instrument

​

Ultra-violet Photoelectron Spectroscopy

and

REELS

24

• UPS is a technique very similar to XPS but which uses UV photons rather than X-ray photons to excite photoelectrons from the surface. As UV photons have lower kinetic energy, the photoelectrons that are detected are from the lower binding energy levels involved in bonding.
• In the laboratory, ultraviolet photons are generated through a gas discharge lamp, commonly filled with helium, although argon and neon are also viable alternatives. Helium gas emits photons with energies of 21.2 eV (He I) and 40.8 eV (He II).

UV photoelectron spectroscopy (UPS)

25

UV photoelectron spectroscopy (UPS)

• Due to the lower energy photons used in UPS, most core-level photoemissions are not accessible, thus limiting spectral acquisition to the valence band region. UPS entails two primary types of experiments: valence band acquisition and electronic work function measurement.

26

UV photoelectron spectroscopy (UPS)

• Ultraviolet Photoelectron Spectroscopy (UPS) Key features:
1. Non-destructive: UPS allows for the characterization of materials without causing damage or altering their chemical composition.
2. Surface-sensitive: It provides valuable information about the outermost layers of a material, making it ideal for studying surface phenomena and interfaces.
3. Energy resolution: UPS offers high energy resolution, enabling the precise determination of energy levels and electronic transitions.
4. Band structure analysis: By measuring the kinetic energies of emitted photoelectrons, UPS can determine the energy distribution and density of electronic states in a material.
5. Sensitivity to valence electrons: UPS primarily detects valence electrons, making it particularly useful for investigating the electronic structure of organic and inorganic compounds.

​

27

Reflected electron energy loss spectroscopy (REELS)

• Reflected electron energy loss spectroscopy (REELS) is a technique used to probe the electronic structure of the material at the surface.
• It works in a similar fashion to Ion scattering spectroscopy (ISS), but in this case, the incident particle is an electron, and it is the scattered electron beam that is measured. The incident electrons can lose energy by causing electronic transitions in the sample, and these energy losses are what is measured in the REELS experiment.
• Properties such as electronic band gaps or the relative energy levels of unoccupied molecular orbitals can be measured. In some cases, it is also able to detect hydrogen, which is not possible with XPS.

28

Example: Energy level characterisation for Ultrathin ALD Ti-doped SnO₂ film vis UPS and REELS

Thank you to our sponsor:

Thank you

​

Questions?

​

​

Have more questions?

​

Yvonne Hora

yvonne.hora@monash.edu