Surface Characterisation

Ion Scattering Spectroscopy (ISS) is a technique in which a beam of secondary electrons is scattered by a surface. The kinetic energy of emitted ions is measured. Depth analysed: 1 – 10 nm

• TRUE

• FALSE

Answer explanation

ISS, is a technique in which a beam of ions (beam of noble gas, usually helium or neon) is scattered by a surface. The kinetic energy of scattered ions is measured; peaks are observed corresponding to elastic scattering of ions from atoms at the surface of the sample.

ISS analyses the Top-most atomic layer.

Attenuated total reflectance (ATR) method uses a X-Ray beam to ionise secondary electrons from the surface to evaluate their chemical nature.

• TRUE

• FALSE

Answer explanation

ATR is one of the most commonly used sampling techniques in recent times. Widely used sampling technique in Fourier Transform Infrared (FTIR) Spectroscopy that enhances the analysis of surfaces and thin films. It allows the examination of samples with minimal preparation by utilising the properties of total internal reflection and evanescent waves to measure the infrared (IR) absorption of a sample. ATR works on the principle of total internal reflection. IR light beam passes through an high refractive index (e.g. diamond crystal) and hits the interface with low refractive index (sample).

Differences in refractive indices à internal reflectance that creates an evanescent wave.

Advantages

•Minimal or no sample preparation

•Applicable to a wide variety of samples

•Compatibility with thick and opaque samples

Disadvantages

•Pressure dependence between sample and crystal

•Less sensitivity to thick or low concentration samples

Scanning Electron Microscopy can be used to visualise the sample morphology of conductive samples.

• TRUE

• FALSE

Answer explanation

Scanning electron microscopy (SEM) - Morphology.

Produces images of a sample by scanning the surface with a focused beam of electrons.

The electrons interact with atoms in the sample, producing various signals that contain information about the surface morphology and composition of the sample.

In the most common SEM mode, secondary electrons emitted by atoms excited by the electron beam are detected using a secondary electron detector.

Nonconductive specimens collect charge when scanned by the electron beam (especially in secondary electron imaging mode) this causes scanning faults and other image artifacts. For conventional imaging in the SEM, specimens must be electrically conductive, at least at the surface, and electrically grounded to prevent the accumulation of electrostatic charge.

Surface modification through plasma treatment has gained widespread importance due to its relatively simple process that is clean, solvent-free, fast, and environmentally friendly.

• TRUE

• FALSE

Answer explanation

Plasma treatment has gained widespread importance due to its relatively simple process, which is clean, solvent-free, fast, and environmentally friendly. In surface plasma treatment, inert gases such as nitrogen and argon are used to dissociate and react with the polymer surface and to change its properties.

(Choose the right option)
Ultraviolet photoemission Spectroscopy (UPS) irradiates the sample surface with UV light (usually helium

discharge). The UV-light is transmitted via a capillary to the specimen's surface, the excitation energy is low compared to the XPS.

• The low energy of UPS photoelectrons is a strong advantage in the study of band structure and surface properties, however it gives UPS a poor resolution.

• The low energy of UPS photoelectrons is a strong advantage in the study of band structure and surface properties, it gives UPS a High resolution.

• The low energy of UPS photoelectrons is a strong advantage in the study of band structure and surface properties, however it gives UPS a poor resolution. But advantageously it does not require vacuum.

• The low energy of UPS photoelectrons is a strong advantage in the study of band structure and surface properties, however it gives UPS a high resolution. But advantageously it does not require vacuum.

Answer explanation

UPS: a surface is irradiated with UV light (usually helium discharge). The UV-light is transmitted via a capillary to the specimen's surface, the excitation energy is low compared to the XPS, thus we will only have contributions from low binding energy levels (valence band). It requires high vacuum.

The low energy of UPS photoelectrons is a strong advantage in the study of band structure and surface properties, however it gives UPS a poor resolution.

(Choose the right answer)
Surface characterisation is an important process that offers an enhanced understanding of the chemical, topographical, and morphological features of surfaces, which play a very crucial role in material properties. Several techniques can be used to study these features:

• Profilometer uses infrared light to measure features on a surface. Producing 3D images of the topography of the sample.

• Profilometer uses an electron beam to measure features on a surface. Producing 3D images of the topography of the sample.

• Atomic force microscopy (AFM) uses a sharp probe that scans the surface of the sample. The ultimate resolution of AFM is

  critically defined and scaled by the radius of the AFM tip, which is about 20 - 50 μm.

• Environmental Scanning Electron Microscopy (ESEM) uses multiple pressure limiting aperture to separate the sample chamber from the column, this allows for non-conductive samples' morphology to be analysed.

Answer explanation

Profilometer - Topography

Uses white light to measure features on a surface, and their operation can be based on a number of different principles.

With a single profilometer scan, a wide range of measurements can be obtained such as 3D waviness, roughness, step height and/or film thickness to advanced fractal analysis and microstructure/particle characterisation.

Atomic force microscopy (AFM) - Topography

The central part of the AFM is the sharp probe that scans the surface of the sample. It is closely monitored in each movement by the piezoelectric materials with a fine subnano-precision. The ultimate resolution of AFM is critically defined and scaled by the radius of the AFM tip, which is about 2 - 5 nm.

Environmental Scanning Electron Microscopy (ESEM) - Morphology

Uses multiple pressure limiting apertures to separate the sample chamber from the column in the ESEM. In spite of the column is still high vacuum, the sample’ chamber pressures is variable. By using this technique, the column and image quality is not damaged. Therefore, moist, wet, hot, dirty, oily, nonconductive samples can be examined in their natural state without modification or preparation.

Atomic Force Microscopy (AFM) allows the visualization of the polymer surface through the use of a probe that scans it, in three different modes.

• For soft materials, the direct contact mode can be use to scan it. Creating a 3D image of the sample' topography without leding to any damage/wear of the surface.

• For soft materials, the non-contact mode can be use to scan it. Creating a 3D image of the sample' morphology without leding to any damage/wear of the surface.

• For soft materials, the non-contact mode can be use to scan it. Creating a 3D image of the sample' topography without leding to any damage/wear of the surface.

• For soft materials, the direct contact mode can be use to scan it. Creating a 3D image of the sample' morphology without leding to any damage/wear of the surface.

Answer explanation

Topography refers to the physical features and relief of a surface. It is a measurement of the vertical and horizontal deviations relative to an ideal flat plane.

Morphology refers to the internal and external structure, arrangement, and shape of the constituent parts of a material. In surface science, it often relates to the arrangement of crystals, domains, or particles at the outermost layer.

AFM can be used to visualise the topography of samples. It may use different modes of analysis:

Contact mode - direct contact with the surface of the sample.

Intermittent mode - (tapping mode). The cantilever is oscillating at its resonant frequency (value depends on the probe type), and with a respective amplitude set-point. There is intermittent interaction with the surface, and the probe experiences both adhesive and repulsive forces during scanning.

Non-contact mode - Mainly dominated by the predominant forces; Van der Waals (attractive) and electrostatic, cantilever is set to oscillate.

For soft materials that can be damaged easily, contact mode is not recommended, has it will lead to damage and wear of the surface. Therefore both tapping and non-contact operation modes are used, this change allows to obtain a more reliable non-damaged topographic image of the samples´surface.

Polymer surfaces can be modified by several methods, for example through patterning. To understand if the

intended pattern was achieved, the surface must be visualise. Describe a possible technique to visualise the topography of the modified surface. Could the chosen method be used for transparent and non-conductive polymers?

Answer explanation

Atomic Force Microscopy (AFM) uses a sharp probe that scans the surface of the sample. It is closely monitored in each movement by the piezoelectric materials with a fine subnano-precision, forming an image of the topography of the sample. One of the advantages of this technique as it does not require any type of sample preparation, therefore the samples’ surface maybe analysed in their natural state, being useful for hydrated, non-conductive and transparent materials.

Most polymeric products owe their applicability, acceptance and behaviour to their interactions with the surroundings. That is defined by its outermost nanometers. It is crucial to precisely map the elemental composition, chemical bonding, and orientation of functional groups only in this ultra-thin surface region (the top 1−10 nm). X-Ray Photoelectron Spectroscopy (XPS) is a method to understand the chemical nature of a polymeric surface. Describe it briefly and what are some of its limitations.

Answer explanation

XPS is a technique that can be used to analyse the surface chemistry of a material.

The solid surface is irradiated with a beam of X-rays and measuring the kinetic energy of electrons that are emitted from the top 5-10 nm of the material. The kinetics energies and intensities of the electron peaks enable identification and quantification of all surface elements (except hydrogen and helium).

XPS uses an hemispherical electron energy analyser: It consists of two concentric conductive hemispheres that act as electrodes that bend the trajectories of the electrons entering a narrow slit at one end so that their final radii depend on their kinetic energy.

XPS is limited to samples able to withstand high vacuum conditions. It is not sensible to H and He.