Non-invasive fabrication of plasmonic nanostructures on dielectric substrates coated with transparent-conductive oxide

. Modern photonics demands for high-resolution (HR) and deterministic lithography on transparent substrates. Thermal scanning-probe lithography (t-SPL) is a mask-less approach that couples a nanoscopic patterning resolution with the possibility to perform morphological characterizations without damaging delicate substrates unlike it happens for other techniques of similar resolution. In order to operate at its maximum performances, an electric bias between the scanning micromachined cantilever and the sample is needed thereby preventing, in principle, the patterning of transparent materials (that are usually insulators). In this work we demonstrate that by intercalating an ultrathin layer of a transparent conductive oxide (TCO) between an insulating and transparent substrate and the polymeric thin layer it is possible to exploit all the benefits of t-SPL also on challenging optically transparent substrates. Taking advantage of this particular lithographic configuration, we were effectively able to obtain a family of different gold plasmonic nanostructures resonating in the spectral range from the Visible to the Near-Infrared. The ensemble of the different resonators shows optical properties that encourage their exploitation in fields like sensing and thermoplasmonics.


Introduction
Nowadays the attention in innovative nanofabrication methods that can perform lithography on transparent substrates without the use of alignment markers and electron or ion beams is great [1].
Thermal scanning-probe lithography is a lithographic technique that allows HR nanopatterning of thermally sensitive films by a sharp conductive probe heated in a controlled way without the necessity of masks or markers.This way a deterministic nanolithography can be combined with pre-patterning and in-operando morphological characterizations of the system all the while maintaining unaltered the properties of delicate substrates (like two-dimensional materials) coated by a sacrificial polymer film [2,3].
Exploiting the sharp silicon tip it is possible to reach a resolution of few tens of nanometres on suitable thermolabile polymeric films [4].
In order to precisely control the nanolithographic process, the t-SPL system applies an electric bias between the scanning head and the sample; this condition is fulfilled by metallic and semiconducting substrates thereby limiting the high resolution nanopatterning onto transparent dielectric substrates.
In this work we demonstrate HR nanolithography capabilities onto optically transparent dielectric substrates by coating the substrates with an ultrathin layer of a transparent-conductive oxide (TCO).
The introduction of this ultrathin TCO layer also enables the morphological characterization of the nanopatterns by high-resolution scanning electron microscopy (SEM), and the optimization of specific t-SPL fabrication recipes which were previously developed for conductive substrates.For different sizes of nanostructures we were effectively able to obtain a family of different gold plasmonic resonators, spanning the spectral range from 700 nm to 1350 nm.In this way we could engineer ordered arrays of plasmonic nanoantennae supported onto dielectric glass substrates.The nanoarrays support Localized Surface Plasmon Resonances tunable over a broadband spectral range from the Visible to the Near-Infrared by controlling the size and/or the shape of the nanoantennae.
The plasmonic response of the different nanostructures is probed by means of transmittance spectroscopy with micrometric spatial resolution [5].

Substrate preparation
The glass substrate is coated with 10 nm of indium-tin oxide (ITO) deposited via radio-frequency sputtering.
On top of ITO a transfer layer of PMMA/MA and a patterning layer of PPA are deposited by means of spin coating.
The substrate is electrically connected with the scanning head of the t-SPL patterning device with an electric-grade copper wire coated with

Patterning
The thermosensitive PPA thin layer is patterned via t-SPL exploiting a NanoFrazor Scholar from Hidelberg Instruments GmbH.

Development, material deposition and lift-off
The PMMA transfer layer is etched with a 5% (v/v) solution of deionized water in isopropanol.A step-by-step atomic force microscopy characterization of the etched patterns enabled us to engineer the best recipe parameters (PMMA/MA thickness and etching times) in order to obtain nanostructures of different sizes preventing both underexposure and collapse issues.
An adhesion layer of Cr and a plasmonic layer of Au are the deposited in ultra-high vacuum by means of molecular beam epitaxy.
The final step is the lift-off procedure in agitated acetone in order to remove the polymeric layers.
Also metal thicknesses and sonication times are adapted the different nanostructure sizes in order to avoid fabrication artifacts and metal detachment problems.

Results and discussions
Two families of plasmonic nanostructures were fabricated: nanodisks and nanorods of different sizes and periodicities in order to exploit at its maximum the geometrical tunability of the plasmonic optical response.
The real geometry of the nanoparticles is evaluated by means of SEM imaging; a SEM micrograph of an array of gold nanoresonators on ITO-coated glass is reported in Fig. 1.

Fig.1. SEM micrograph of an array of gold nanostructures obtained by t-SPL on ITO-coated glass
This analysis made it possible to assess the enlargement of the nanostructures caused by the fabrication process: it ranges from 25% for structures with a nominal diameter of 100 nm to 55% for structures bigger than 150 nm.
A custom-made spectro-microscopic setup was employed to investigate the plasmonic properties of the arrays of nanostructures.This setup benefits of two spectrometers coupled in tandem so that the spectral range spans from 400 to 2240 nm and the measurement spot diameter is about 20 µm [6].
The optical response of the arrays of nanostructures is dominated by the dipolar LSPR modes for all the geometries while for structures larger than 100 nm also multipolar modes are present, even if less intense.
The good quality factors of resonators smaller than 100 nm prove the reduced polydispersity in size and therefore the quality of the process making this lithographic solution an efficient tool for thermoplasmonic heating, sensing and strong-coupling applications.