MA1 BRUFACE - Chemical & Materials Engineering - Summary with notes

SURFACE
TREATMENT OF
MATERIALS




SOMI Alex

2016 | 2017

, I. Introduction

- The surface is defined as the region near the surface where the properties
changes, the interphase between two phases. The size of a surface depends on
what we want to measure:
• Outer mono layer: 0.1nm
• Thin surface film: 0.1-100nm
• Thin film: 0.1-10 m
• Thick layers: >10m

- The interaction radiation-sample has to be selected in the right way depending on
what we want to detect. The radiation can be electrons, ions, photons (= primary
beams) and the detection for the analysis use the secondary particles.

- The use of high vacuum is often used to avoid surface contamination. It allows to
work on the sample during several minutes before forming a dirt monolayer. UHV is
also used to avoid the strong absorption of reflected particles (electrons for example)
before reaching the detector (highly reactive with air).

- Sometimes, it’s also possible to work in situ to study the kinetics of surface in real
time. To do so, we work with photons because they are less absorbed than electrons
or ions.

II. Local Probe Methods

A. Introduction

- The local probe methods use a sharp probe scanning over the surface and
detecting changes in the surface morphology. The resolution of these methods is
atomic (angstrom).

- These methods are often used because they don’t need UHV or sample
preparation.

B. Scanning Tunnelling Microscopy (STM)
1. Principle of tunnelling

- The STM works on the tunnelling effect. If we apply a difference of potential
between the sample and the probe, we have a probability  0 to pass electrons from
the probe to the sample. The intensity of the tunnelling current will follow an
exponential decreasing with the distance probe/sample.

𝐼 ∝ exp (−𝑘𝑥)

2. The probe and the sample

- The sample and the tip have to be a conductive material in order to use STM. The
tip has to be as sharp as possible (ideal single atom) → importance of the tip
structure.


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, - The distance tip/sample has to be about the atom’s diameter to have a good
resolution. To do this, the probe is moved thanks to piezoelectric crystals. These
crystals have separate positive and negative charges (→polarization) and can be
deformed (order of m) by applying an electric field.




3. STM modes

- We can work with several modes:
• Constant current: measure of the distance variation tip/sample to keep
the current constant.
• Constant distance: measure of the current variation to keep the
distance constant.
• Potential scanning: applying a potential to see how many electrons we
have on the surface → can give an information if the states are filled or
not → information on the material we have at a certain place.

C. Atomic Force Microscopy (AFM)
1. Principle of AFM

- The probe is either attracted or repulsed from the sample and the force is
measured. By knowing the force, the distance can be deduced → information on the
topography.

- The interaction probe/sample are Van der Waals interaction → no electrons
sharing!




2. The probe and the sample
- The sample can be non-conductive with this method. The tip has a mechanical
design (spring) to deduce the distance tip/sample by measuring the force (Hook’s
law).
- The force is measured with a laser beam send to the probe and deviated on a
photodetector.

3. AFM modes



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