This document discusses silicon nanocrystals. It defines a nanocrystal as a crystalline nanoparticle with at least one dimension less than or equal to 100 nm. It explains that silicon nanocrystals can emit light even though bulk silicon cannot. The document discusses two methods for preparing silicon nanocrystals: electrochemical etching of silicon wafers and pulverization. Electrochemical etching uses hydrofluoric acid to dissolve silicon and form a porous structure containing silicon nanocrystals. Pulverization produces silicon nanocrystal agglomerates in powder form.

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*What Are Nanocrystals???
*How Are Nanocrystals Prepared???
*How To obtain Silicon
Nanocrystals???
1-Electrochemical Etching
2-Pulverization
*References

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*A nanocrystal is a crystalline nanoparticle.
*Some sources define it as any single crystalline
nanomaterial with at least one dimension ≤ 100 nm.
*others define it as a nanoparticle with any kind of
crystalline structure.
e.g., multiply twinned
configurations.

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*Silicon nanocrystals can provide efficient light
emission even while bulk Silicon can't!
*Nanocrystals embedded in solids can exhibit
much more complex melting behavior than
conventional solids!

5. *The traditional way :
Prepare nanocrystals of a new material involved
choosing molecular precursors, surfactants, and
solvents using optimized reaction conditions
causing the atoms to self-assemble into
monodisperse nanocrystals.
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*The newer and simpler strategy:
Use of preformed nanocrystals as templates
and chemical transformation to change the
composition.

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*Silicon nanocrystals are prepared using
electrochemical etching of silicon wafers.

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*The procedure takes place under normal
atmosphere, in Teflon chambers with electrical
current flowing through the electrolyte.
*The final product are oxide-covered silicon
nanocrystals.

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*Disadvantage of this method: The natural
tendency of oxide-covered silicon nanocrystals
to form strongly bound large agglomerates of
tiny nanocrystals!

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1: Electrochemical etching
*The electrochemical etching, is based on the use of
hydrofluoric acid.
*Fluorine ions attack silicon surface, causing its
dissolution into hexafluorosilicate ions.

10. *This dissolution forms voids in the silicon wafers,
resulting in a porous structure with small chunks of
silicon consisting of nanocrystals between the pores.
*Ethanol, which is added in order to increase
wettability and facilitate the penetration of the
etching solution into the tiny pores.
*The etching procedure yields a layer of
interconnected silicon nanocrystals.
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2: Pulverization
*Obtaining the sample in the powder-like form,
consisting of larger agglomerates of small silicon
nanocrystals. The powder can be stored for a
long time in air without any detrimental effect
on its properties.

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*Synthesis of Water-Soluble Blue Photoluminescent Silico
http://research.ncku.edu.tw/re../articles/e/20091113/2.html
*Nanocrystal
http://en.wikipedia.org/wiki/Nanocrystal
*Nanocrystalline Silicon for Nanophotonics
http://nanotech.fzu.cz/26/index.php?file=1
*Journal of the Korean Physical Society, Vol. 42, February 2003
Synthesis of Silicon Nanocrystals
By: Cho Joon Wo & Lee So Jin
*Nanocrystal Synthesis
By: Nathanael Smith

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