You will find here a brief overview of some of the projects I have been involved in. I selected the ones I consider the most important and interesting, which allowed me to develop a higher understanding of physical phenomenon and technological development.

Ultra-Fast Laser Sintering

Recently, printed electronics has attracted considerable research interest due to its potential of development for flexible display, antennas, sensors and wearable electronics.

Functional inks are one of the most suitable materials for those applications. Main barrier for the application of printed electronics are due to their high cost, instability and high sintering temperature. This heat treatment, that is sintering, is an important step in printing of metal nanoparticle inks to remove dispersant and auxiliaries, which will seriously affected the nature of textiles of fabrics as high temperature leading to deformation, hardened hand feel and yellowing, especially those thermal sensitive fibers. Furthermore, high temperature also causes oxidation of some metallic nanostructures and results in poor conductivity.

The use of a laser offers an alternative sintering technique, instead of heating. These technique is capable of sintering at a relatively low processing temperature with a relative cheap equipment, simple procedures and easily-scale production.

Recently I started assembling a new instrument aiming to perform ink sintering based on a laser scan system. The high power of the laser coupled with a high-fast scan set-up provides the starting point for an new sintering technique capable of low temperature processes with a simple and easily-scaled instrumentation.

This equipment can open the way to print on new substrate such as polymer, paper and thin film.

In the following video is presented the instrument I am creating with the key components.

This project is a collaboration within the MASSIVE project, EPSRC funded.

Hyperspectral Thermal Imaging

When you are working on imaging sensors, in order to maximize the spatial resolution, the single elements into the detector have to be minimized. But if you are producing a multi-spectral analysis you are interested in having as much filters as possible to maximize the number of frequencies investigated. I have studied the possibility to perform Hyperspectral Imaging in the IR region. This spectral range is quite important for several applications in biological analysis, sensing and security applications.

Nowadays is impossible to perform hyperspectral imaging in the IR region of the spectrum using compact devices. This is due to the fact that the commercial fabrication techniques to produce IR filters cannot scale them to a single pixel dimension. In order to scale down the filters at a single pixel dimension, new technologies are required.

I developed the fabrication technique to create this new peculiar array of coaxial antennas. The scaffold of an hallow polymeric antenna is coated by sputter coating with gold (a). The gold from the substrate is removed with a Reactive Ion Etching process (b). A uniform layer of insulator is deposited with the Atomic Layer Deposition on the entire sample (c), this layer is going to be the spacer between the antennas and the mirror layer. A mono layer of silane is evaporated on the sample to produce an oleo-phobic surface (d), this step is mandatory for the next step. On the sample is spun a layer of polymer (e), thank to the oleophobicity the polymer doesn't produce any meniscus on top of the antennas. As last step a highly vertical gold evaporation is performed (f).

I characterized the filters with FTIR investigation. The first spectrum (a) shows the transmittance behavior of a specific set of parameter of a theoretical simulation (black dotted line) compared with experimental data. The red line represents a filter with a size of 50 by 50 um^2 and it has a high transmittance and a good agreement with the simulation. The different high of the peak is mainly due to fabrication imperfection. The most remarkable result is represented by the blue line coming from a filter with 10 by 10 um^2 size, about 2 by 2 time the operational wavelength and about a common pixel size.

But to be used as hyperspectral filter the tunability is fundamental, and here it is introduced the second plot (b), where the transmission peaks of different set of parameters are represented. The lines represent a simulation and the squares and dots the experimental data. It is clear that changing the lattice period and the length of the antennas is easy to tune the filter to scan the entire IR frequency range.

Within this project I demonstrate that this design works as an efficient filter also at a single pixel dimension in a broad range of IR frequencies. I hope that this filter could open the way to an hyperspectral thermal imaging with compact devices.

AFM Bead Lithography

The optical lithography has a huge paradigm: if you want to increase the resolution of your final frame, the cost of the instrument you need rises dramatically.

However, there is a well known technique able to perform high resolution optical lithography with barely no cost, which is bead lithography. To briefly describe it, a layer of densely closed packaged micro-spheres are placed on top of the sample. Each bead acts as a micro-lens and focuses the incoming radiation in a focal volume, called nanojet, that can reach sub-wavelength dimensions. The weakness of this technique is that it doesn't permit to produce nothing different than disks with an hexagonal space arrangement.

To conduct the experiments, I specifically modified a standard tipless cantilever, producing a hosting cavity with a Focus Ion Beam in order to place the micro-sphere inside it.

The experiments are then performed with a set-up that I have created: a microscope is customized to couple an AFM scanning probe micro-manipulator with a common 405 nm laser and both a coaxial upright and an inverse optical microscope.

One of the strengths of this approach is the simplicity with which it can be implemented on various existing instruments. The generation of an arbitrary pattern using microbead lithography only requires an AFM that provides a scan pattern generator. Because this software tool is already present in several standard types of equipment, writing arbitrary patterns on photoresists using our microbead lithography approach is highly achievable.

Moreover, I compared the performances of this new technique with a standard high-resolution optical lithography instrument (Laser writer DL66 Heidelberg). The results showed how the bead lithography can reach a higher resolution with a higher speed keeping the cost of the entire instrument cheaper.

I presented a novel high-resolution low-cost lithographic system based on a microbead embedded in a tipless AFM cantilever; this solution can be implemented in a broad range of applications. I hope that this approach can advance the microbead lithography method towards highly versatile and commercial methodologies, offering increased resolution and a higher degree of freedom compared to standard commercial instruments. The presented approach can be easily implemented on existing relatively low-cost AFMs instruments that are already present in many research laboratories to offer high-resolution lithography capabilities combined with cost-effective tools.

This new technique has a great potential to introduce rapid high-resolution microbead lithography as a new standard technique in several laboratory environments due to its low cost compared with modern commercial lithography instruments, as it is has been explained in the scientific magazine Nanowerk.

Solar Rectenna

Light rectification addresses the conversion of visible light into DC electricity. The mechanism is such that electromagnetic radiation couples to an antenna and induces an oscillating (AC) electric signal, which will be rectified by a diode into a DC current, the whole constituting a rectenna, producing power across a connected load. This approach has been demonstrated and used with success in the low frequency range of the electromagnetic spectrum, particularly in the microwave range, but until now, our experiment is the first proof-of-concept of visible light rectification on a single antenna.

In this study, these challenges were addressed by introducing a new methodology for the production of nano-antennas and the development of a versatile setup for the consistent test of a diode’s material stack, allowing therefore the study of occurring fundamental processes and the creation of practical design criteria for nano-rectennas.

My contribution to this project was to design the fabrication process to realize the nano-antennas and to carry out the fabrication as well. Moreover, I fabricated the substrate, finding the suitable stack of materials able to perform as high frequency diode.

The rectenna was fabricated starting from a standard cantilever with a initial coating of 7 nm fo Ti and 15 nm of gold (a). The tip of the cantilever is then cut with a Focus Ion Beam (b). On the created plateau, a platinum nano-cone with the designed geometrical parameters is grown using Gas Injection Deposition (c). At this point a new layer of 15 nm of Au is placed by sputter coating (d). The sample receives a final annealing step. This method guarantees the fabrication of nano-cone with 5 nm apex radius and few hundreds of nm of base and high with an extremely high reproducibility.

The gold tip is the first sub-component of the MIM configuration, the second is the Insulator-Metal2 (IM2) junction. On a fused silica substrate is evaporated 50 nm of Ti, then a 2 nm layer of TiO2 is deposited with the Atomic Layer Deposition creating an extremely thin conformal layer. The deposition was performed at 200 C and a long cooling ramp is performed. The resulted substrate was found with a roughness of about 0.6 nm.

Once the rectenna is mounted on the AFM and inserted into the circuit and shined with the laser, we could actually measure the generation of current coming from the conversion of the light on the surface of the tiny tip.

The video I have prepared presents a standard experiment in which a single-tip MIM is used. But In order to create a potential rectenna device it is mandatory to scale up the system to a multi-tip MIM, and we hope that our work could lead to a new generation of light converter even more efficient than the most efficient semiconductor solar cell.

MEMs Varifocal Mirror

The main goal of this project is to create a MEMS varifocal mirror in order to overcome this problem. Varifocal mirrors are deformable mirrors, whose curvature can be dynamically changed, allowing to control the focal length of the laser without mechanical displacement. Those mirrors are specifically designed to control the focus with a high number of degrees of freedom, and they typically allow to control the focal length and several aberrations. This makes them very compact, fast and simple to control, while providing large optical power range.

In this project I designed the fabrication process and I realized the mirrors. The process starts with a silicon wafer coated with 500 nm of super low stress low-pressure chemical vapour deposition (LPCVD) silicon nitride on both sides (a). A chromium layer of 250 nm is deposited on the bottom of the wafer and patterned to expose a circular window of 4 mm of diameter (b). Using the patterned chromium layer as a mask, the bottom nitride layer is removed with reactive-ion etching (RIE) (c), then the silicon bulk is etched with a BOSCH deep reactive-ion etching (DRIE) process, which allows drilling as circular hole in the wafer with vertical walls (d). The wafer is cut in single mirrors (e). After that, the chromium layer and the remaining silicon are wet etched to release the silicon nitride membrane (f). The top of the membrane is spun with two layers of photoresist and patterned to expose a circular window aligned with the released membrane (g). Then a titanium-gold layer (7 nm of titanium and 200 nm of gold) is evaporated on top of the wafer (h). Finally, the resist is removed with solvent, leaving the gold coating only on the circular membrane (i).

Nano-Fluidic Deposition

The advent of analytical techniques with extremely low limits of detection led to dramatic progresses in the field of nucleic acids sequencing. I collaborated within the FET Open PROSEQO project, where we built upon current state-of-the-art sequencing technologies to develop novel proof-of-principle technologies for high-throughput protein sequencing and single molecule DNA/RNA sequencing.

This huge project has been divided in sub-tasks:

• a new sequencing technology that utilizes plasmonics to enhance the optical detection and to control the molecules movement by electrical trapping;
• a novel approach of plasmonic enhanced FRET and SERS spectroscopy for sequencing of proteins;
• a rigorous analytical model to reconstruct the exact sequence from the signals recorded;
• a single molecule sequencing device that can perform sequencing of both nucleic acids and amino-acids in one functional unit.

My duty for this project was to scale down to the micro/nano scale the electrophoresis technique, in order to study the possibility to control the single particle positioning with nanometer precision and drive selectively the particles into nano-structures. The nanometer precision of the particle positioning dramatically increases the single molecule detection, which is critical for the success of the project.

I created a PDMS microfluidic chamber to control with high precision the deposition and displacement of diluted material, such as Quantum dot or flakes of Molybdenum disulfide, onto the nanostructures fabricated on a membrane, leaving the membrane completely clean.

This result is remarkable because this technique is easily scalable for a high throughput performance keeping the precision high and the cost low.

Moreover, this study led to a better understanding of the single molecule motion into highly viscous and conductive medium; these results are explained in the Data Section.