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[ Last edited by caosw2001 on 2008-7-28 at 09:12 ]
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4Â¥2008-07-27 19:41:25
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ÕâÊÇLudovico¸øÎҵĻØÐÅ

thanks for writing me about your experiments. I am always looking forward for
ways to help fellow chemists. Thanks also for the kind words.

I have a couple of observations

1) from the absorption spectra you have you can see that there is no peak and
that you have a gradually increasing background absorption so:

a) the absorption you see might come from scattering (dust, excess PbCl2 which
you haven't precipitated or aggregates of nanocrystals) - proposed solution:
try to centrifuge the dispersions for at least 10 minutes just before
measuring absorption and check if you see any difference
b) the absence of the peak might come from very large polydispersity (poor
quality of the sample) - proposed solution: use different batches and sources
of oleylemine (I always used oleylamine from the 100G package from Aldrich) -
other oleylamines give worse results. Maybe you can send me a TEM image of
your sample.
c)the fact you don't see the peak can also come from the fact that you are
measuring in the wrong range of frequencies: you say that your nanocrystals
are 10 nm (they are VERY BIG - it is very hard to get PbS nanocrystals of
that size with my reaction). Try to get smaller nanocrystals (shorter growth
times) and measure their absorbance
d) chloroform and chlorurated solvents are BAD for PbS (use toluene, hexane or
THF if you can)
e) the quality can be bad also because you use too much ethanol to precipitate
the nanocrystals - proposed solution: use acetone which is way better -
acetone allows you to completely precipitate all of excess PbCl2 immediately,
without having to wait for weeks
f) the solutions you prepare might be far too concentrated. when you measure
absorbance you should be able to see through the 1 cm cuvette
g) poor quality material can also come from excessive cleaning cycles - limit
yourself to two cleaning cycles

This is what I can think of right now. If you can, you should send me some
more information so that I can help you better (a representative TEM image
and a very detailed list of the synthesis and purification procedure you use
would help me identify the problem)

Keep me posted

Cheers

Ludovico
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5Â¥2008-07-27 19:59:33
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about Ludovico

Age: 29
Citizenship and Residency: Italian citizenship, Canadian permanent residency
Education: University of Parma, Italy ¨C University of Toronto, Canada
Background: Materials Science, Nanochemistry, Materials Chemistry
Skills: Nanocrystals, Photonic Crystals, Mesoporous Materials, Nanowires, Colloids
Students Supervised: 5 (leading to 5 joint publications)
Instruments: STEM, SEM, XRD, ICP, XPS, TOF-SIMS, Plasma reactors
Awards and Scholarships: 10 (including Governor General Gold Medal, 2 MRS Graduate Student
Awards and ACS DIC Young Investigator Award)
International Conference Contributions: 24
Invited Talks at Conferences: 5 (including ACS, CSC, MRS, Particles)
Invited Lectures: 3
Books: 1
Publications in International Journals: 13 (average IF = 8.48, h-index = 6, citations ~ 76)
i believe i can fly !
6Â¥2008-07-27 20:02:53
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