Silicon nanostructures are good templates for the deposition of plasmonic metals to fabricate substrates showing activity in surface enhanced Raman scattering (SERS). Porous silicon formed by electrochemical anodic etching of monocrystalline silicon wafers is more commonly used while silicon nanowires grown by metal assisted chemical etching are also utilized. The main advantage of such substrates is that they provide deposition of polydisperse films consisting of densely arranged metal nanoparticles (NPs) from liquid solutions. The resulting films exhibit properties in SERS-spectroscopy typical for substrates made by other more expensive and complicated methods of nanoengineering. Majority of papers report on metal films consisted of quasispherical NPs, but some works demonstrates that it is also possible to form metal rods, dendrites and thorns of nanoscaled dimensions.
This work is aimed at studying how the type and level of doping of the original silicon affects the formation process and the morphology of the metal nanostructures, and hence their optical properties, which determine SERS-activity.
We deposited silver (the strongest plasmonic metal) on silicon nanostructures grown on n- and p-type wafers of varying doping level. It should be noted that, depending on the doping conditions, both photoluminescent and non-luminescent silicon nanostructures were obtained.
In addition, we studied the deposition of silver on silicon nanostructured particles obtained by thermal reduction of silicon dioxide extracted from the horsetail. Such nanoparticles are composed of only silicon atoms, i.e. are characterized by intrinsic conductivity.
It was found that the p-type of the initial substrate inhibits growth of metal nanostructures due to absence of free electrons. At the same time, there is a growing trend towards elongated and dendritic nanostructures.
The n-type silicon leads to the formation of quasispherical metal NPs and agglomerates. Remarkably, nanocrystals of intrinsic conductivity that have sizes of several nanometers are shown to have a chemical sustainability. As a result, they are not oxidized during immersion into the solution containing silver ions and silver is not deposited.
Influence of silver NPs on the photoluminescence of silicon nanostructures and the SERS-activity was studied. It is shown that the SERS-activity of quasispherical metal NPs on nanostructured n-type silicon rises as mass of the deposited silver increase until NPs coalesce into agglomerates.
On the other hand, the SERS-activity of single dendrites grown on p-type silicon nanostructures is higher than that in quasispherical NPs. However, as the amount of dendrites and the thickness of the dendritic layer increase, the activity decreases.
The application perspectives of the SERS-active substrates based on photoluminescent silicon nanostructures for simultaneous bioimaging and molecular identification are also discussed.