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Absorption band and absorption peak?

  1. Mar 19, 2013 #1
    The absorption spectra of some materials appear as the absorption bands (for example: TiO2 powder) while the other appear as the absorption peaks (for example: Quantum dots). Please explain why they are different?
  2. jcsd
  3. Mar 19, 2013 #2


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    Staff: Mentor

    How do you distinguish between bands and peeks?
  4. Mar 19, 2013 #3
  5. Mar 20, 2013 #4
    I don't think you are using the word "band" correctly. A "band" can have a maximum.

    You probably mean "low energy threshold". For example, I was working with spectra from an impurity in a semiconductor. The emission intensity showed a relative peak at a photon energy of 105 meV. However, the absorptivity associated with this impurity showed a low energy threshold at 105 meV.

    The "threshold" means that the impurity did not absorb radiation with a photon energy less than or equal to 105 meV. However, the impurity did absorb radiation with a photon energy greater than 105 meV. The absorptivity of the impurity increased monotonically with photon energy. Therefore, there was no relative maximum in absorptivity. The absorption spectrum showed a step at the threshold, not a pointy hat.

    You are asking why some transitions are associated with peaks and some with thresholds. Both spectra are associated with electrons that are being boosted in energy.

    In spectra associated with peaks, one energy level is "catching" all the electrons. The electrons lose energy, but there is a bottleneck between the initial energy and the ground state. The narrow peak is caused by transitions from this bottleneck to the ground state (or some lesser excited state).

    That is why my emission spectrum showed a peak at 105 meV. I boosted the electrons very high in energy. However, the bottom of the conduction-band caught the electrons before they got to ground state. The final transition was from the conduction band to the impurity state.

    In spectra associated with thresholds, there is no bottleneck. There is a continuum of energy states that can receive the electron. The density of this continuum generally increases with photon energy. The threshold is associated with the bottom of this continuum. Therefore, the absorption goes up.

    That is why my absorption spectrum showed a threshold at 105 meV. The conduction band is a continuum. However, there is no bottleneck to absorption. So the bigger the photon energy, the bigger the absorption.

    Note it was the same impurity in both cases. Even the same energy (105 meV). However, the emission process allows some time for the electron to be caught by a bottleneck. The absorption process did not allow any time for the electron to be caught.

    This is an old article. However, I posted it on ResearchGate. If you go to the ResearchGate website, maybe you can get a free or inexpensive copy. I don't know if they charge, so I won't promise anything. The article does refer to both spectra of the 105 meV impurity.

    The reference is:
    "Native defects in undoped semi-insulaing CdSe studied by photoluminescence and absorption," by D. L. Rosen, Q. X. Li and R. R. Alfano, Physical Review B31, 2396-2403 (15 February 1985).
    Last edited: Mar 20, 2013
  6. Mar 21, 2013 #5
    Thank you Darwin123 for your discussions.
    May be I had a mistake in using the scientific terms. What I want to say is some materials absorb all photons which their energy are higher than the band gap of material while some materials absorb strongly the photons which have certain energy. This means if the photons energy are higher than the certain energy (and band gap), the materials absorb lesser
  7. Mar 21, 2013 #6
    That sounds more or less what I was trying to say. Of course, I was skipping some of the qualifications and details.

    There is a trade-off between simplicity and accuracy. If one tries to make an explanation simple enough to understand, one often has to ignore "exceptions to the rule". When one ignores "exceptions to the rule", one often runs into ambiguity. The least ambiguous answers often sound the most pedantic.

    Anyway, I think the best word for the "band gap" sort of spectrum is "threshold". I generally use the word "band" for a spectral feature with a "peak". This is the way the words were used in that article that I cited.
    Last edited: Mar 21, 2013
  8. Mar 21, 2013 #7
    I have seen the word "shoulder" used for "thresholds". The words have slightly different meanings, but often refer to the same type of features. A shoulder is usually a change in the derivative of a spectral function without a change in sign. A threshold usually refers to an sudden increase in the value of a function without a concomitant drop.

    The words threshold and shoulder refer to step-like spectral features. They don't specify physical mechanisms that cause the spectra features. The word "band gap" refers to the electronic structure of the material. You don't always want to imply that you know the structure of the material when all you have is the spectrum.

    I found one article claiming that the absorption feature which characterizes TiO2 is caused by a "band gap". I would call that feature a "threshold" rather than a "band gap" feature.

    “Clearly, the absorption spectra show the onset of absorption ( determined by the linear ex-
    trapolation of the steep part of the UV absorption toward the base line ( 8)), …350 nm, corresponding to a band-gap energy of”

    The absorption spectrum of quantum dots often have a peak and a threshold feature superimposed. The threshold feature is associated with the continuum of states associated with the bandgap of the material, while the narrow band is caused by an exciton energy level just below the band gap.

    Note the emission spectrum of quantum dots show a narrow band associated with the threshold alone. That exciton is what I called the “bottleneck” in a previous post. Electrons high in the continuum settle down into the exciton state, and then jump to the ground state. The emission spectrum shows the exciton to ground state transition.

    See the following link on quantum dots. Note that the absorption spectrum of the QD has a threshold that overlaps the narrow band. The narrow band is caused by an exciton caused by quantum confinement.
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