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Nanometer Resolution in Two-Beam Direct Laser Writing Lithography

Final Report Summary - NANOR 2BDLW (Nanometer Resolution in Two-Beam Direct Laser Writing Lithography)

The scientific activity during the first reporting period can be divided into two main objectives:
1.- Investigation of the photophysics of some free radical photoinitiators where the triplet absorption to a higher triplet state was proven to account for the depletion of the triplet intermediate state. The decrease of the triplet concentration below a certain threshold by using an inhibition beam permitted to improve the spatial resolution and diminish the linewidth of the polymerized structures. It was intended to study the photodynamics of the higher triplet state to demonstrated the reversibility of this depletion mechanism.
Pump-probe and pump-dump-probe spectroscopy was utilized to investigate the decay of the higher triplet state. These transient experiments were carried out in EtOH solutions at high concentrations of the corresponding photoinitiators (isopropyl thioxanthone (ITX), 7-diethylamino-3-thenoyl-coumarin (DETC), 4-4´-bis-(diethylamino)benzophenone (BDEBP)). The pump-probe experiments showed the absorption peaks of the singlet and triplet states and the kinetics of the intersystem crossing. Once this information is obtained, the specific wavelength and time delay for the dump beam in configured in the pump-dump-probe experiments. This third beam is used to efficiently excite the triplet state to a higher triplet state, measuring the transient absorption of this higher triplet state (with the probe beam) and its deactivation pathway. However, we were not able to detect the transient signal of the higher triplet state with our experimental conditions. We attributed this to two main reasons: the first one, the low concentration of triple state that we were to populate with the pump beam, and the second one, the low extinction coefficient of the triplet-triplet absorption hampering the population of a measurable concentration of the higher triplet state with the probe beam. Summarizing, the absence of transient signal from the higher triplet state in the pump-dump-probe experiments prevented the investigation of the kinetics of its deactivation and therefore the expected reversibility of this mechanism is still a matter of research.
2.- Exploration of novel molecules as cationic photoinitiators due to the several advantages of this polymerization process respect to the free radical one. Moreover, it is expected to reduce the “memory effect” which diminishes the writing resolution in this technique due to the addition of radicals from adjacent irradiations. Thus, a back electron transfer should regenerate the parent photoinitiator from the photogenerated radicals that do not react with the monomer.
In a recent article, the C60 fullerene was employed as photoinitiator in a macroscopic polymerization of a cationic resin. Based on that article, the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), a fullerene derivative with higher solubility in organic solvents, was selected to carry out cationic polymerization together with an electron accepting molecule, the AgPF6. To validate the use of PCBM in multi-photon absorption techniques, the two-photon absorption cross-section was calculated by using the z-scan method. A value of 0.65 GM was determined, much lower than that of other photoinitiators but still high enough to allow for generation of enough cations at the used excitation powers. Both molecules were mixed with an araldite cationic resin and irradiated with a femtosecond laser beam focused with a 1.4 NA objective lens to fabricate the structures. High-quality lines were fabricated with this system as shown by the SEM images. To investigate the possible improvement in the writing resolution, lines separated by different distances were prepared with this system but also with a conventional cationic photoinitiator. The AFM profiles of the different samples proved the improvement in the writing resolution from 600 to 450 nm, for the conventional and PCBM-based photoinitiator systems, respectively.
I have thereby presented a new cationic photoinitiator that improves the writing resolution, but it is still necessary to carry out pump-probe spectroscopy to investigate more on the nature and kinetics of the back electron transfer reaction. This new approach to enhance the writing resolution can be employed in other systems with much better mechanical properties to achieve real nanometer resolution that can compete with the more sophisticated lithography techniques.

We have discovered a new family of molecules that can act as cationic photoinitiators. Fullerene derivatives are commonly known as electron acceptor but can also work as electron donors. The radical cations formed after a simple electron transfer reaction can trigger polymerization. The reversibility of this type of reactions (back electron transfer) allows to improve the writing resolution when using these photoinitiators.
We have also established a one-pot method to fabricate a phase hologram through precise modification of the thickness of a photopolymerized resin. This approach takes advantage of the thermal effect provoked by the interaction between high peak-power femtosecond lasers and acrylic resins, resulting in an controlled expansion of the resin. The main characteristic of this method is the simplicity to fabricate the holographic voxel, since in one simple writing process is possible to control the shape and height of the feature.
Moreover, the controlled expansion of the resin has a collateral effect in the photophysics of a fluorescent dye encapsulated in the resin. The dye inside the resin forms aggregates and therefore there is a strong quenching of the fluorescence. The expansion of the resin due to the thermal effect provokes the di-aggregation of the dye since the dye-to-dye distance is much longer. Thus, the lifetime of the monomer form of the dye is longer and therefore it is possible to selectively functionalize the surfaces of the structures with a higher fluorescent signal.
Finally, the use of the two-beam approach in lithography when using the ITX molecule as the photoinitiator and a CW laser beam at 642 nm as the inhibition beam allowed the decrease of the linewidth from 600 to 450 nm.


- Participation in the IV Retreat of the Nanophysics Department of the Italian Institute of Technology, held in Sestri Levante (Italy), 29th September to 2nd October 2014. This was an open forum for discussion about innovative fields of research (graphene flagship, super-resolution microscopy, neuroplasmonics) aimed at encouraging the researchers to establish long-term collaborations in the field. Dr. de Miguel discussed the possibility to functionalize the polymer structures fabricated with the lithography technique with metal and semiconductor nanoparticles. As a result of this, a collaboration was established with Dr. Intartaglia to dope polymerized lines with 20 nm diameter Ag and Au nanoparticles.
- Participation as teaching staff in the Nikon Imaging School held in Genova, December 2nd-5th 2014. Dr. de Miguel was in charge of the section “Coupling an AFM to a STED microscopy”. During the three days of the school, forty participants were taught the main advantages of combing the nanometer resolution AFM technique with the STED microscopy that provide useful information about the chemical composition of the samples.
- Supervision of an internship student, Marjukka Elmeranta, for 6 months, July 1st 2014 to January 31th 2015. During this time, she has been involved in the nanofabrication of nanostructures by using the two-beam approach with the ITX molecules as photoinitiator. In parallel to this, she also used STED microscopy to determine the size of the structures and comparing it with the values obtained with the AFM technique.
- Dr. de Miguel has established a fruitful collaboration with Dr. Duocastella, a researcher from the same group, with the aim of functionalizing the polymerized structures with fluorescent molecules but also with conductive materials, as silver nanorods or graphene sheets.