Introduction to Surface Analytical Techniques

Introduction to Surface Analytical Techniques

Surface analytical techniques are specialized analytical techniques that are used to gather information about the surface of a substance.

Most of those techniques involve bombarding the surface with a form of radiation such as electrons, photons, ions, or neutrons and then collecting the resulting emitted radiation (electrons, photons, ions, neutrons) using a detector.

How surface can be defined?

The top layer of atoms (or molecules) of a substance that immediate interfere with other phases such as gas, liquid, or solid phases can be defined as the surface.

The structure and chemistry of that top layer of atoms (or molecules) are determined by the atoms or molecules immediately below that top layer. Therefore, the surface can be defined as the top 2 to 10 atomic or molecular layers of a substance. The thickness of the surface layer lies between 0.5 nm and 3nm.

In many applications, surface films are applied to materials to obtain different purposes such as,

• to protect,
• to lubricate,
• to change the optical properties of the surface.

Most of those surface films are in the range of 10 nm to 100 nm (could be much thicker in some cases). So, a depth of 100 nm from the top layer can be used to describe the surface because this area is responsible for determining the properties of the surface.

Beyond this 100 nm region, those layers are much more appropriate to describe the bulk solid state properties of the material.

Depending on this, the surface can be divided into 3 regions.

1. The top surface monolayer
2. The first ten or so atomic or molecular layers (0.5 nm – 10 nm)
3. The surface film (10 nm – 100 nm)

Structure of the surface

To study the surface of a solid material, surface analytical techniques should have the potential to distinguish the properties of all three layers of the surface as well as distinguish the surface from the bulk.

Estimation of the number of atoms in the surface layer

Let’s consider a metal cube of 1 cm³ volume. It can be estimated that a surface that is 1 cm² contains 10¹⁵ atoms. So, within the 1 cm³ volume of the cube, it can be estimated that there will be roughly 10²³ amount of atoms.

When considering the surface-to-bulk ratio, can be calculated as follows.

The importance of surface studies

• Development of surface that has corrosion resistance
• Development of water-repellent materials
• Development of non-stick surfaces
• Change the optical properties of surfaces
• Development of catalytic surfaces for industrial applications
• Applications in nanoscience

Why surface analytical techniques should be highly sensitive?

When looking at the above numbers it is obvious that the number of atoms (molecules) in the surface of the material is very small. So, examining the surface layer of atoms or molecules analytically and distinguishing the properties and the structure of the surface layer from the rest of the bulk material is not an easy process.

Most of the surface analytical techniques have the ability to analyze (probe) a surface area of 1mm². Within this area, there are about 10¹³ amount of atoms. This can be considered a very low concentration when compared with other chemical analysis processes. To conduct that kind of analysis instruments and methods must be highly sensitive.

Things will get even more demanding and much higher sensitivity will be required when needed to analyze surface-influencing chemical species such as additives, and contaminants. Generally, those chemicals have much less concentration than surface atoms.

Let’s consider some other specific applications such as nanoparticle (nano surface) analysis, monitoring the distribution of catalyst across the surface of the support, monitoring homogeneity of a protective coating, monitoring optical property, and monitoring the effect of a drug on a cell. In most cases, those surfaces are in the micrometer range (some of them are much smaller than that). When that kind of analysis is required, that requires high spatial resolution.

So, conducting surface analysis demand higher surface resolution and sensitivity. To obtain that highly sensitive, specialized analytical instruments and equipment must be used.

Common surface analysis techniques

• Augur Electron Spectroscopy (AES)
• X-ray Photoelectron Spectroscopy (ESCA / XPS)
• Ultraviolet photoelectron spectroscopy (UPS)
• Low energy Electron diffraction method (LEED)
• Reflection high energy electron diffraction (RHEED)
• Electron Energy Loss Spectroscopy (EELS)
• Secondary Ion Mass Spectroscopy (SIMS)
• Scanning Tunnelling Microscope (STM)
• Scanning Electron Microscopy (SEM)
• Infrared Reflection Absorption Spectroscopy (IRAS)
• Sum Frequency Generation Spectroscopy (SFGS)
• Transmission Electron Microscopy (TEM)
• Inelastic neutron scattering (INS)

Information obtained from surface analysis

The intention of surface analysis is to get an understanding of the properties, structure, composition, and reactivity of a surface. To obtain that, information about the surface can be collected using surface analytical techniques. These are the types of information obtained from the surface analysis.

• Chemical structure of the surface
• Physical topography of the surface
• Chemical composition of the surface
• Atomic Lattice structure
• Molecular bonding Information at the surface
• Oxidation State of Atoms at the surface

There are many techniques to analyze surface as mentioned above. However, all of the surface information cannot be obtained by one single method. To gather all the information about the surface, several different techniques must be used.

Characteristic of surface analytical techniques

In most techniques, surface analysis processes must be conducted in a vacuum. The reason behind that is those techniques use energy sources (radiation) like electrons, ions, neutrons, and photons. If there are any molecules in the gas phase, they could interfere with the radiation (by reflecting, refracting, absorbing, or scattering radiation).

Also, to prevent surface contamination by the adsorption of particles in the gas phase, some instruments must be operated under ultra-high vacuum (<10⁻⁹ mmHg).

Surface sensitivity of surface analysis techniques

The surface analysis technique should have the ability to obtain information within the defined depth of the surface layer. A technique that has a good surface sensitivity should gather information from as close to the top surface monolayer. The surface sensitivity of an instrument is a measurement of the extent of the depth it collects the information.

In most methods, when the surface is bombarded with a source of radiation, the bombarded atoms or molecules will emit a form of radiation and detectors use this emitted radiation to gather information. When a surface is bombarded with radiation, that radiation has the ability to penetrate some layers of the surface. As a result, the detected radiation is generated in those deep layers. So when incident radiation penetrates the deeper layers, the information obtained from those methods has less surface specificity.

Surface damage during the surface analysis process

To obtain information about the surface, it is necessary to interfere with the surface somehow. As mentioned above, most techniques use bombarding the surface with a radiation source. That will affect the physical and chemical properties of the surface being analyzed. That will change the surface while the analysis happens. In most cases, the detected signal (radiation) could be interfered with by this surface damage.

It is important to understand this process because if not, the information generated from the surface could be a reflection of the surface damage (may not obtain the characteristic information of the surface).

The extent of surface damage based on the type of incident radiation

Photon < Electron < Ion