Charge Referencing Insulators
Charge Referencing (Correcting) of BEs from Insulators
Charge Referencing is also known as Charge Correction. They are the same process to correct or reference BEs from non-conductive materials. Charge correction is NOT charge compensation or charge control. There are various charging problems and charge referencing problems.
Use “Hydrocarbon” Type C (1s) BE at 285.0 eV to “Charge Reference/Correct” BEs
from Non-Conductive Materials (Insulators)
If there are problems or overlaps or the sample has no carbon
use the BE of a “Well-Known Material” to “Charge Reference/Correct” BEs
of your sample / material.
For Example: Use one of the major BEs from SiO2, or Al2O3, MgO, or a Polymer (eg PET, Teflon)
Charge Referencing Problems
There are numerous causes of problems with Charge Referencing.
Horizontal Differential Charging is the most common type of differential charging. A similar type of charging is due to Vertical Differential Charging.
When you analyze materials that have a thin layer (<5 nm) of non-conducting material (eg native oxides) on top of a conductive material (eg a metal) that is grounded, we should normally turn the Flood Gun OFF to avoid the type of Vertical Differential Charging shown below. If you do need to keep the Flood Gun turns ON, then it is best to use as low a voltage as possible with low current, again to avoid or minimize vertical differential charging.
Differential Charging is one type of Charging Problem
A Question:
Is 284.8 eV the “correct” C (1s) BE for hydrocarbon type carbon for all materials?
All the time?
The following two (2) examples reveal that 284.8 or 285.0 eV is probably not the “correct” C (1s) BE to charge reference non-conductors that have Strong Surface Dipole Moments. By analyzing the native oxide of Al and Mg metals, the hydrocarbon C (1s) BE from them, and comparing them to BEs derived from pure Al2O3 and MgO, we can see that there is something unexpected happening. After reviewing the Synchrotron AlOx Growth data shown here, we can better understand what is happening.
Analyzing Al2O3 – Problems?
Analyzing MgO – Problems?
Hydrocarbon C (1s) BEs on Naturally Formed Native Oxides and
Hydrocarbon C (1s) BEs
that Formed on Ion Etched Metals after 10 Hrs in UHV
After analyzing these BEs from the Hydrocarbon C (1s) we found that the average (mean) value from naturally formed native oxides appeared at 285.4 eV, which is 0.6 eV above the widely accepted 284.8 eV value. Keep in mind that the 284.8 eV value is not a standard and has never been been fully studied. Beginners are trained to “accept” and use one of the various values when they are first learning XPS. Those values range from 284.2 eV to 285.2 eV.
The average (mean) BE from the Hydrocarbon C (1s) peak that resulted after the ion etched metals sat in UHV for >10 hr is 284.9 eV.
Median Value of C (1s) BE of Hydrocarbon Moiety on Old Native Oxides
Median Value of C (1s) BE of Hydrocarbon Moiety on Freshly Ion Etched Pure Metals
Historical Values used for C (1s) BE of Adventitious Hydrocarbon Moiety
Differential Charging
There are two types of Differential Charging:
Horizontal Differential Charging is the most common type of differential charging. A similar type of charging is due to Vertical Differential Charging.
Horizontal Differential Charging
When you analyze materials that are thick non-conductive materials that have a rough surface (eg powder), have an odd shape (eg a toothpick), or have a mixture of conductive and non-conductive material side-by-side, then the surface might develop Horizontal Differential Charging.
If the charge neutralizer (low voltage beam of electrons, flood gun) is not properly aligned or does not have enough voltage or emission current, then the surface will also show horizontal differential charging. An example of horizontal differential charging is shown here. This drawing also shows that you can use a Charge-Control Mesh-Screen to level out the non-uniform electric field by having a grounded mesh-screen sitting 0.5-1.0 mm just above the surface of the sample.
This overlay shows the effect of differential charging that is only 30 eV in strength. The SiO2 coating is thick, but has some channels in the film, that allows electrons from below to propagate along grain boundaries to reach the surface, but the number of electrons is not enough to fully neutralize the surface.
This overlay shows the total effect of having a surface that suffers differential charging. There is not only a positive (+) charge shift tof ~30 eV, but the number of photoelectrons that can escape are reduced by 4X the full emission from a properly charge compensated surface.
Horizontal Differential Charging
Vertical Differential Charging
When you analyze materials that have a thin layer (<5 nm) of non-conducting material (eg native oxides) on top of a conductive material (eg a metal) that is grounded, we should normally turn the Flood Gun OFF to avoid the type of Vertical Differential Charging shown below. If you do need to keep the Flood Gun turns ON, then it is best to use as low a voltage as possible with low current, again to avoid or minimize vertical differential charging
Two (2) Diagrams that depict Differential Charging
The Charge-Control Mesh-Screen is a Solution to Differential Charging
Flood Gun Alignment (XY) is an Integral Part of Charge Control\Compensation
Test of Alignment using Teflon
Flood Gun Alignment (XY) is an Integral Part of Charge Control\Compensation
Test of Alignment using Polypropylene
End-of-page