Multi-photon patterning of photoactive o-nitrobenzyl ligands bound to gold surfaces.

We quantitatively investigate lithographic patterning of a thiol-anchored self-assembled monolayer (SAM) of photocleavable o-nitrobenzyl ligands on gold through a multi-photon absorption process at 1.7 eV (730 nm wavelength). The photocleaving rate increases faster than the square of the incident light intensity, indicating a process more complex than simple two-photon absorption. We tentatively ascribe this observation to two-photon absorption that triggers the formation of a long-lived intermediate aci-nitro species whose decomposition yield is partially determined either by absorption of additional photons or by a local temperature that is elevated by the incident light. At the highest light intensities, thermal processes compete with photoactivation and lead to damage of the SAM. The threshold is high enough that this destructive process can largely be avoided, even while power densities are kept sufficiently large that complete photoactivation takes place on time scales of tens of seconds to a few minutes. This means that this type of ligand can be activated at visible and near infrared wavelengths where plasmonic resonances can easily be engineered in metal nanostructures, even though their single-photon reactivity at these wavelengths is negligible. This will allow selective functionalization of plasmon hotspots, which in addition to high resolution lithographic applications would be of benefit to applications such as Surface Enhanced Raman Spectroscopy and plasmonic photocatalysis as well as directed bottom-up nanoassembly.

Light-Directed Patchy Particle Fabrication and Assembly from Isotropic Silver Nanoparticles.

We demonstrate the creation of anisotropic patchy silver nanospheroids (AgNSs) using linearly polarized UV light and a photo-uncaging o-nitrobenzyl-based ligand, which anchors to the AgNSs by two gold-sulfur bonds. Exposure to a 1 J/cm2 dose of UV light induces a photo-uncaging reaction in the ligand that reveals a primary amine on the surface. By using linearly polarized UV light, we meter the exposure dose such that only the poles of the nanoparticle receive a full dose, limiting the photo-uncaging reaction primarily to the particle's plasmonic hot spots. We reveal this anisotropy by preferentially adhering negatively charged gold nanospheres (AuNSs) to the AgNSs' poles by using the electrostatic attraction between them and the positively charged primary amines generated by photo-uncaging. When the assembly is performed onto silver particles that are immobilized on a substrate, it results in nanoscale structures with a strong tendency to align with the polarization of the exposing light. This manifests in polarimetric spectroscopy as a linear dichroism aligned with the polarization direction.