sample workfunction

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Thomas Mueller replied on Fri, Jun 4 2010 2:02 PM

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Hi Alberto,

Yes, Kelvin probe microscopy can tell you differences in electrostatic potential. With some assumptions you can deduce workfunction values. And yes, on a MultiMode with NanoScope IIID this mode is available. But please do not hesitate to elaborate on your question so I can better understand what you are asking.

Best,

Thomas

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a_savoini replied on Mon, Jun 7 2010 10:04 AM

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Hi Thomas,

I think that I should use HOPG as a reference material ( 4.6 eV ) and use KPM to have a value of CPD ( between HOPG and  PtIr5 tip ).

Then, using the same parameters of KPM, I calculate  CPD of the sample ( for example Al  or  ITO ).

Now I could have the workfunction of the sample, using  the two CPD.

Is it correct ?

Thank you,

Alberto

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Chunzeng Li replied on Tue, Jun 8 2010 1:12 AM

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Dear Alberto,

The procedure you outlined is perfectly fine. The simpler way yet is to use a tip that has a known and well defined workfunction, for example, Pt-coated tip (such as OSCM-PT from Veeco Probes ). Let the tip assume the workfunction of Pt ~5.5V , then the workfunction of the sample is simply 5.5V- surface potential (contact potential difference).  More generally:

Wsample = Wtip - Surface Potential (contact potential difference between sample and tip measured with Kelvin probe force microscopy).

Note:

1. In the above equation, the DC bias is applie to the tip. If DC bias is applied to the sample, then it rewrites as Wsample = Wtip + Surface Potential;
2. In ambient, there is uncertainty in KPFM measurements arising from adsorbed molecules from ambient on the surface, oxidation of surface, surface states etc.
3. The measurement also has dependency on tip-sample separation.

Chunzeng

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N-ElHinnawy replied on Thu, Jun 17 2010 11:51 AM

Alberto:

The question you asked is unfortunately a lot more complicated than people want to admit.  While extracting qualitative data from a KPFM image can be helpful, in order to extract quantitative data (like workfunction) you have to have many qualifiers in place.  The most important part of the process is to know exactly what surface you are imaging.

The most accurate statement you can make about KPFM data is that it tells you the DC offset voltage the SPM had to apply in order to nullify first order oscillations in the cantilever.  Factors that determine what this DC offset voltage mean:

1) dielectric, semiconducting, or metallic
The term "contact potential difference" is really only valid when you are imaging a metal, and even then there are environmental qualifiers which could negate this.  When imaging something other than a metal, the DC offset will not represent the work function of the material.

2) environmental conditions
KPFM in vacuum has a much different meaning than a KPFM in air.  You would be surprised what sort of difference realtive humidity can make on measurements on the same sample.

3) sample being measured
a KPFM image of Si will not give you the work function of the material, just as a KPFM image of Ti, Cr, or Al won't give you the work function of those metals either.  Care has to be taken to understand exactly what is on the surface and how much, because thickness will play a role as well.

The short answer is that extracting quantitative data from KPFM measurements is a dangerous game to complete accurately and ethically, even for simple workfunction measurements.