Photoelectric effect, photoelectron spectroscopy (XPS) and scanning photoelectron microscopy (SPEM) are explained in this short lecture. The photoelectric effect occurs when the inner shell electrons of the sample atoms are kicked out by high energy electromagnetic radiation. These electrons, that were kicked out, are called photoelectrons and their energy depends on the energy of the exciting radiation, the electrons initial binding energy and on the work function. When having a radiation source with well defined energy and measuring the energy of the emitted electrons, it is possible to get information about the samples composition and chemical state. In the case of photoelectron microscopy the exciting beam is focused into a narrow spot on the sample surface. The sample is then moved in such a way that the spot moves row by row across the sample surface and as a result photoelectrons are emitted from each irradiated spot. By collecting these photoelectrons, it is possible to map the composition or chemical state across the selected sample area (for example 100 x 100 microns area on a 2 x 2 cm sample). Photoelectrons can escape the material only from near the surface and this means that these methods are very surface sensitive, which makes them extremely useful for surface studies. This also means that the surfaces need to be very clean (cleaned with ion bombardment before measurements) and therefore ultra-high vacuum is needed.
Atomic force microscope (AFM) is a powerful tool that is used to study materials by scanning over the surface with a very sharp tip. When a sharp tip approaches a surface then first Van der Waals attractive forces apply that pull the tip closer to the surface and therefore also bend the cantilever. When the tip is close enough then electrostatic repulsive forces apply. The bending of the cantilever is monitored with a lazer beam that is focused on the cantilever and reflected into the detector.This method allows to obtain greatly magnified high resolution 3D images of the studied substrates (even atomic resolution is possible). The main working modes are contact, non-contact and tapping mode. In then case of contact mode the tip is in direct contact with the material. This is suitable for studying hard surfaces. In the case of non-contact mode the tip is vibrating close to the surface. This mode is used for studying sticky and soft surfaces. In the tapping mode the tip vibrates with a greater amplitude and briefly touches the sample at its lowest point of the trajectory. This method is useful for obtaining a “real” image of the studied surface as the tip penetrates the thin film of water that is always present on substrates when measuring in open air. There are also other types of scanning probe microscopes where different information can be obtained from the sample. For example a if one uses a thermocouple as a sharp tip for scanning then it is possible to study heat distribution on microscopic electronic devices in order to detect possible spots where oveheating occurs. In the case of scanning tunnel microscopy an electric potential is applied between the tip and the sample, which causes the movement of electrons from one to other. By measuring the tunneling current, it is possible to obtain valuable information about the state of the surface. Even atomic resolution is possible in STM and therefore can be used to study novel materials such as graphene. It is also possible to use magnetic needles or thin optical cables as tip for scanning over the studied surface.
The scanning electron microscope is a powerful tool used by many scientists for studying and developing materials. This device uses electrons instead of photons and that allows the operator to visualize even nano-scaled surface details. Therefore it is very useful for also studying corrosion processes and creating efficient thin coatings which have a thickness of less than a micrometer.
In the video above you will see the working principle of this microscope and how it is operated.
The electrons are generated in the electron gun on top of the microscope. There are different source types but one of the oldest used is a tungsten filament that is heated over 2400 degrees celsius by passing a current through it. At this temperature electrons emit from the filament and are pulled down the column with the help of an anode. The operator selects the voltage that accelerates the electrons. It can be up to 30 kV in conventional scanning electron microscopes. The accelerated electrons are scattered and need to be focused – this is done with the help of multiple focusing lenses. These lenses also tune the amount of electrons that reach the sample. For example by broadening the electron beam, the electrons are absorbed by the column and dont reach the sample. In order to scan across the sample row by row, the electron beam needs to be moved in such a manner. This is achieved with the help of scanning coils that are placed below the focusing lenses. The final focusing of the beam is done with objective lenses located below the scanning coils. In order to get an image of the substrate the surface is scanned row by row with the electron beam. In each spot signals like secondary electrons, backscattered electrons or characteristic X-Rays are generated. By detecting the signal emitted from each scanned spot, an image of the surface is generated. So for example by detecting secondary electrons a secondary electron image is generated.
Secondary electrons are generated when the primary beam electrons kick out electrons from the substrate atoms. In that process primary beam electrons lose energy as it is transfered to the secondary electrons. These secondary electrons generaly have low energy and escape only from near the surface, giving a good image of the substrates topography. The electrons are collected with a special detector that has a positively charged collector for pulling the negatively charged electrons towards it.
Backscattered electrons are primary beam electrons that are scattered back in a similar direction as they interact with the substrate atoms nucleous. In that process they dont lose much energy and can be emitted even from deep layers of the substrate. Therefore they carry the bulk information of the sample and generated images are not completely topographic. The electron yield strongly depends on the atomic number of the substrate. For example regions on substrate with higher atomic number appear brighter on the image as more electrons are backscattered in these regions.
Characteristic X-Rays are generated when sample atoms exited by primary beam electrons are undergoing a relaxation process where the inner shell vacant spot is filled with an outer shell electron. The emitted radiations energy depends strictly on the atoms number and on the electrons binding energies that are involved in this process. Therefore this signal can be used to detect different elements and do quantitative element analysis.
For a real SEM study the sample first needs to be prepared – it has to be dry, conductive and with a suitable size. Non-conductive samples can be coated with a thin (couple nanometers) layer of metal (Au, Pt..), which allows SEM studies as they conduct away the heat and negative charge caused by electrons. The sample is moved into the microscope through an airlock or the main chamber door and placed on a stage with a special holder. The stage can move in any desired direction (x,y,z, rotate, tilt). The sample is then moved under the electron column to a suitable working distance. After focusing and other adjustments the first electron image can be taken.
Element microanalysis can be done in several ways but first an electron image of the sample is obtained. The most common way to do element analysis is selecting a characteristic spot on the sample and then bombard it with the narrow (couple nm wide) electron beam. As a result X-Rays are emitted from the couple micrometer wide interaction volume. This gives information from a very localized area but in some cases an average composition of the material is needed. For that a larger area (lets say 50 x 50 microns) is bombarded by electrons and the X-Rays emitted from that area are detected. It is also possible to map the distribution of elements on a larger area by scanning across the surface with the electrons row by row and collect X-Rays from each scanned point. Based on the detected X-Rays an image is created that shows the distribution of certain elements.
In order to see what is inside a material a focused ion beam (FIB) is used to make a cross-section or a thin lamella. The cross-sections or lamella are then studied with electrons at a suitable angle by tilting it with the help of the stage.
Scanning electron microscopes generally work in a high vacuum in order to prevent surface contamination and electron or x-ray interactions with the gas in the chamber which would affect the quality of the image. However there are also specially designed environmental scanning electron microscopes (ESEM-s), that work in low vacuum. The ESEM can therefore even be used to study living cells or bacteria.