Growth mechanism of photoreduced silver nanostructures on periodically proton exchanged lithium niobate: Time and concentration dependence

Photodeposition of metallic nanostructures
onto ferroelectric surfaces, which have been chemically patterned using a
proton exchange process, has recently been demonstrated. By varying the
molar concentration of the AgNO3
solution and the illumination time, one can determine the initial
nucleation sites, control the rate of nucleation and the height of
silver nanostructures formed, and study the mechanisms by which these
processes occurs. The nanoparticles are found to deposit preferentially
in the boundary between ferroelectric and proton exchanged regions, in
an area proton exchanged via lateral diffusion under the masking layer
used for chemical patterning, consistent with our previous results.
Using a short illumination time (3 min), we are able to determine that
the initial nucleation of the silver nanostructure, having a width of
0.17±0.02µm and a height of
1.61±0.98nm, occurs near the edge of the reactive ion etched area
within this lateral diffusion region. Over longer illumination times (15
min), we find that the silver deposition has spread to a width of
1.29±0.06µm, extending across the entire lateral diffusion region. We report that at a high molar concentration of AgNO3 (10¯² M), the amount of silver deposition for 5 min UV illumination is greater (2.88±0.58nm) compared to that at low (10¯4M)
concentrations (0.78±0.35nm), however, this is not the case for
longer time periods. With increasing illumination time (15 min),
experiments at 10¯4 M had greater overall deposition, 6.90±1.52nm, compared to 4.50±0.76nm at 10 ¯² M. For longer exposure times (30min) at 10 ¯² M the nanostructure height is 4.72±0.59nm, suggesting a saturation
in the nanostructure height. The results are discussed in terms of the
electric double layer that forms at the crystal surface. There is an
order of magnitude difference between the Debye lengths for 10¯² and 10¯4 M solutions, i.e., 3.04 vs. 30.40nm, which suggests the Debye length plays a role in the availability of Ag+ ions at the surface.