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Binding energy and work function of a solid?

  1. Oct 30, 2006 #1
    Can someone explain the difference to me? Is it correct that the binding energy is the energy needed to bring an electron to the fermi level, and the work function it the energy needed to bring an electron from the fermi level to the vacuum level?

    What are typical values of the binding energy and the work function? Is one of them much larger than the other?

    Thanks!
     
  2. jcsd
  3. Oct 30, 2006 #2
    binding energy is a term usually spoken of in context of atoms ie. the energy to to bring a core electron from an atom into vacuum. binding energies and work functions are rather pointless to compare because the difference between atomic electrons and the ones occupying the bands is very distinct but binding energies are usually orders of magnitude larger.
     
  4. Oct 30, 2006 #3

    ZapperZ

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    Well, I think loosely used, Repetit's definition of binding energy is OK. I know we use it in photoemission spectroscopy to designate the energy of the photoelectrons we extracted as measured with respect to the Fermi energy. So this corresponds to the definition given.

    The work function "effective definition" is also as given in the OP, only it gets a bit messier if one tries to actually define it carefully. For example, in many instances, the work function includes the "image potential" of the charge that just barely made it out of the metal with almost no energy, but gets attracted back to it due to the image charge. So the "work function" is an over-estimation of the true work function, but only by a very slight amount that makes no difference in many instances.

    Unfortunately, for some of the things I had to study, such differences do make a difference. So that's why I had to carefully investigate these things. Things get very tedious and difficult once we try to pry open the intricate details of a phenomenon.

    Zz.
     
  5. Oct 31, 2006 #4
    Thanks for the help! Good to know that I was not too wrong on the meaning of these terms :-)
     
  6. Nov 5, 2006 #5
    I agree with your definitions but in the case of the work function (WF) it is better to say : "the work function it the energy needed to bring an electron from the fermi level to the vacuum level AND KEEP IT THERE"

    What i mean with the addendum "KEEP IT THERE" is that once you bring an electron "outside" a material into the vacuum, there is going to be an image potential that wants to pull back the electron towards the material's surface. So a WF must also include this : not only is it the energy to get an electron outside the metal (ie get it out of the conduction band in the case of metals for example) and to [ii] put the electron into the vacuum (ie overcome the surface potential) but also to [iii] keep the electron at the vacuum level (ie overcome the image potentials).

    Another thing, when studying interfaces of different materials (like a metal/ high k dielectric interface) there is also the concept of "apparent WF". For example, in the metal/high k dielectric (eg HfO2) interface, the metal apparent WF is the metal WF close to the interface. This metal WF will not be equal to the pure metal WF (when the metal is NOT in contact with the dielectric : so no interface) because in the interfacial region, the electrostatics that determine the WF-value are determined by both the metal AND the dielectric. Such topics are very interesting and important to, for example, current ongoing CMOS device technology. This is an example of solid state physics bein' used in CMOS technology and this is exactly what i am doing in my PhD.

    regards
    marlon
     
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