6/8/13 Electrically Controlled Nanoparticle Synthesis inside Nanopores

Paper:

Electrically Controlled Nanoparticle Synthesis inside Nanopores

Kimberly Venta, Meni Wanunu,† and Marija Drndić*

Keywords: : Nanopores, nanoparticles, silicon nitride, synthesis, transmission electron microscopy, metallic nanostructures

Dead Body: Controlled placement and synthesis of NPs is difficult, and the smaller NPs are limited by some processing methods

Prior work: Doesn't seem to be a lot, except using nanopores for translocation of single molecules in very small pores

Solution: Use nanopores drilled into SiN as reaction sites for NP synthesis driven by electric field, and measure synthesis with changes in electricity.

Background: Limiting reaction volume to form other materials: organic particles in solution, high
aspect ratio nanowires, and electrofunctionalized micropores as well as studies of precipitation-induced ion current fluctuations in nanopores and related mathematical modeling

What are the most interesting results: 

• The NP forms in a single nanopore. It fills the entire width of the nanopore and is self-limiting when the precursors can't reach each other though the pore anymore.
• The dI/dt derivative of the reaction current was measured to show the nucleation, growth, and the filling growth characteristics.
• The voltages were a few hundred mV, and nA of current.
• they created a model for the growth of the NP that has some constants such as the particle growth constant and constraint constant (rep. difficulty for reagents to meet in the narrow pore), and gives a function for the radius vs. time.
• Use of an alpha-lipoic acid capping on the Au made the NP form much faster (there was almost no time delay) and suggests that it helps with the nucleation of the NP. 
• From TEM analysis, the AuNPs were single xstalline and didn't show a lot of gold formation surrounding the single NP
• They did a nanopore array of 4 different pores, and found a stepwise reduction in current as the nanopores were closed.  without the addiction of alpha lipoic acid, there was a lot of extra gold accumulation as well connecting the 4 NPs.

What are the most interesting discussions: 

• The concentrations of the precursors was chosen to minimize NP formation time without going to unsafe amounts of hydrazine
• The reaction is self limiting because when the reagents can't reach each other anymore, the reaction stops
• They added a thicker layer of SiO2 and a silicone elastomer to reduce the capacitance and therefore decrease the current spike when the switched the voltage
• There was a long delay between switching the voltage and the formation of the NP.  They suggested that during this delay, smaller particles or coagulants might form in parts of the pore.  These would need to be minimized for future applications.
• Decreasing the KCl molarity (and therefore ionic strength) reduced this time delay by 1-2 orders of magnitude
• 
  

Materials used: 

KCl (5mM - 1M) as electrolyte
HAuCl4 5 mg/mL solution for 2.94 mM
hydrazine 0.0312 mM


How this applies to my work:

• Does this actually need to be SiN, or can anything be used?
• Why do the pore sizes need to be so small, or can they be bigger?
• If they can be bigger, than BCP can be used to etch the pores and make tons of NPs all over.  
• If there is a huge array of NPs, will the yield be 100%?
• Would HIBL be faster at making these nanopores?
• Can the BCP film itself be the actual nanopore thinfilm?

Applications:

Cited - transport measurements, self-assembly, and catalysis, expansion to arrays of NPs with individual addressing from electric field, expand to other materials and metals, TEM study of individual nanoparticles

My own - applications of gold NPs as antenna arrays? Nucleation for VLS? Can the NPs then be placed down on another surface?