Alternative nano-lithographic tools for shell-isolated nanoparticle enhanced Raman spectroscopy substrates

Chemically synthesized metal nanoparticles (MNPs) have been widely used as surface-enhanced Raman spectroscopy (SERS) substrates for monitoring catalytic reactions. In some applications, however, the SERS MNPs, besides being plasmonically active, can also be catalytically active and result in Raman signals from undesired side products. The MNPs are typically insulated with a thin (∼3 nm), in principle pin-hole-free shell to prevent this. This approach, which is known as shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), offers many advantages, such as better thermal and chemical stability of the plasmonic nanoparticle. However, having both a high enhancement factor and ensuring that the shell is pin-hole-free is challenging because there is a trade-off between the two when considering the shell thickness. So far in the literature, shell insulation has been successfully applied only to chemically synthesized MNPs. In this work, we alternatively study different combinations of chemical synthesis (bottom-up) and lithographic (top-down) routes to obtain shell-isolated plasmonic nanostructures that offer chemical sensing capabilities. The three approaches we study in this work include (1) chemically synthesized MNPs + chemical shell, (2) lithographic substrate + chemical shell, and (3) lithographic substrate + atomic layer deposition (ALD) shell. We find that ALD allows us to fabricate controllable and reproducible pin-hole-free shells. We showcase the ability to fabricate lithographic SHINER substrates which report an enhancement factor of 7.5 × 103 ± 17% for our gold nanodot substrates coated with a 2.8 nm aluminium oxide shell. Lastly, by introducing a gold etchant solution to our fabricated SHINER substrate, we verified that the shells fabricated with ALD are truly pin-hole-free.


Supplementary Information
Step No.
Step The thickness of the coating can be adjusted by changing the cycle number which repeats the steps from step 1 to step 16.It is important to note that after each pulse, a wait step of 5 seconds is added as a hold time, during which the vacuum system is shut off, the continuing gas flow increases the pressure in the system, but the precursor remains for longer at the sample surface.This ensures that the precursor material has enough time to cover all the surface of the nanostructures.This modification is typically used for high aspect ratio structures to ensure conformal deposition of the material.15 ms pulse time was chosen based on the characteristics of the machine in terms of reaction volume, gas flux and the necessary reaction pressures.The rather short pulse time is still long enough to ensure that there is no lack of precursor material during the deposition process.The subsequent N2 purging step was used to clean the chamber and remove excess precursor material.

Figure
Figure S1: BARC+PFI-88 nanodots fabricated on a silicon wafer by displacement talbot lithography

Figure S2 :
Figure S2: TEM image of the SHINs with a zoom-in on an individual nanoparticle, showing the SiO2 coating around the particle.The scalebar in the image is 100 nanometer.

Figure
Figure S4: a) Raman spectrum of the pyridine adsorbed on the gold nanoparticles.Two peaks at 1008 and 1030 cm -1 can be observed.b) Same as in a), but now for the chemically synthesized shells on the gold nanoparticles.Four spectra are plotted with an offset, to show the variance of the signal and the difficulty to observe or exclude the pyridine vibration.

Figure
Figure S7 (a): Gold reference sample with 17 ALD deposition cycles of Al2O3 subjected to gold etchant test.

Figure
Figure S7 (b): Gold reference sample with 19 ALD deposition cycles of Al2O3 subjected to gold etchant test

Figure S8 :
Figure S8: AuNP@SiNC coated with 19 ALD deposition cycles of Al2O3.Missing gold nanoparticles can be visualized on top of the nanocones

Figure S10 :
Figure S10: Voigt peak fitting of the 1362 cm -1 peak from the Raman spectrum depicted in Figure 4b.The area between 1300 and 1400 cm -1 was used, with two different Voigt line shapes for the two distinct Rh6G vibrations.

Table S2 :
Overview of the (average) signal area of the 1362 cm -1 peak with the respective variance over the measured 100 pixels of 1 µm