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




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 




Analyzing MgO



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.