Background & Objective
In the era of the Internet of Things, electronic sensors perform as extensions of various human perceptions, which can expand novel sensory aspects and even detect various meaningful signals that have been neglected in the past. We can expect that whenever a new sensor technology is developed, it can help us obtain more data. After analysis and computation, new applications can be produced to meet our needs for safety, health, and happiness. Compared with the satisfaction of entertainment needs, how to apply the Internet of Things technology to meet the needs of human health is still a virgin land waiting to be developed. In recent years, many research teams, including us, have been exploring this virgin land, hoping to bring brand-new perception technology, so that people can more effectively and conveniently maintain their health, early detect the disease warning signs, and enjoy high-quality safe and healthy sports.
Figure 1. Schematic view of the process that we will develop involving preparation of metal oxide precursors solutions, thin films and curing by laser. Targeted applications are metal oxide sensors prepared at room temperature on various substrates such as glass, plastics, nanocomposites, textiles. Low-cost, versatile integration process makes it possible to develop new applications in the field of Internet of Things (IoT).
In the past, our bilateral team has developed several low-cost highly-sensitive gas sensors that can be used to detect different components in the breath and moved towards non-invasive health warning applications. Among them, the detection of exhaled ammonia was already verified by the Taiwan team in cooperation with medical doctors in Taiwan to conduct human clinical trials. The exhaled ammonia can be used as an indicator for early screening of chronic kidney disease, which is suitable for rural medical screening or for the early detection in high-risk groups. At the same time, we have also seen the importance of fine motion detection in aging warning or in enhancing the training efficiency of competitive sports (such as skiing, kayaking, cycling, etc.). Therefore, since two years ago, we boldly proposed the possibility of using near-infrared laser irradiation to replace the conventional high temperature thermal annealing in fabricating sol-gel metal oxide semiconductor devices. After verification, this photo direct write/anneal technology can indeed break through the production of electronic components on flat substrates. It can be more delicately fabricated on many types of substrates.For example, we have successfully demonstrated light and gas sensors on 3D printed material or on the extremely soft food plastic wrap.
This project includes state-of-the-art fundamental aspects of design, preparation, processing and characterization of sol-gel materials prepared in thin film form. The core of the project is the understanding and optimization of light-material interactions, at the molecular, nanometric scale, to prepare functional nanocomposite materials. The main application domain is that of chemical or physical sensors, for health, sport and well-being. These ambitious objectives will be achieved by combining the complementary skills of the two Taiwanese and French teams. This project is therefore a continuation of our collaboration but introduces new developments in terms of materials and applications thanks to the common experience developed in the last years. From the point of view of applications, we foresee applications in the biomedical field, in sports and well-being.
Scientific Solutions
Figure 2. The different options for laser curing of sol-gel thin films.
Basically our project relies on the combination of sol-gel chemistry and laser curing to obtain functional thin films on different substrates. Metal oxide thin films can be prepared from solution of metal oxide molecular precursors. Usually, a thermal curing is applied to obtained the final functional film. Even though the annealing conditions are considered as “chimie douce” conditions, temperatures as 300°C to 600°C are usually required to obtain suitable properties.
These objectives combine fundamental concepts of design and preparation of functional materials by liquid process.
- A disruptive route for room-temperature preparation of micro-nano-patterned electronic materials
The main challenge of this project is the fabrication of solid-state metal-oxide micro-nano-structures with a low thermal budget and simple technologies. Indeed, the metal oxide semi-conducting materials are usually prepared by sputtering or sol-gel route but always with thermal annealing to obtain suitable electrical properties. Moreover, for sputtering, an additional photolithography step is required to define the microstructures. This multi-step process is time-consuming, hardly compatible with unconventional substrates and the properties of the metal-oxide thin film may be affected by the etching process.
So, today, no satisfactory simple technological solution is available to achieve a room temperature lowcost solution compatible with flexible polymer substrates. Here, NIR laser writing is proposed to directpattern the oxide semiconductor microstructure and also to cure the material in a single step. The interest of NIR laser curing is that it is possible to deliver the energy needed to cure the sol- gel precursors into
metal oxide thin film at a very localized scale. The key-point is to obtain high absorbance in the sol-gel thin film, and low absorbance of the substrate (glass, polymer). The last point explains the interest of NIR (800-1000 nm) for which these materials are enough transparent.
Based on preliminary results, we like to explore and optimize in this project several routes in this field of NIR laser curing of sol-gel thin films:
Figure 3. Potential species to allow efficient laser curing of this films to obtain metal oxide thin films with suitable electronic properties.
- NIR dye for photothermal treatment.
Solutions from Zinc-oxo clusters (ZnOC) will be used thank to the background of previous common works. ZnO has indeed interesting intrinsic electrical properties. These properties can be easily tuned by doping with In, Ga, W, Al. ZnOC solutions (based on Zn-Methacrylate and Zn-nitrate) also showed good ageing stability and films can be easily prepared from these solutions by spin-coating.
Main objective will be to define the suitable NIR dyes that can act as efficient photoheaters. The absorbers will be chosen in the family of indocyanines for which there is a wide range of commercial products with different functional groups that allows tuning the absorption spectrum and yield of nonradiative relaxation for heat production. The dyes will be characterized in terms of absorption spectrum
in the solution and thin film to optimize the light-material interaction. IR140 and HITC (Aldrich) will be investigated first as they proved to be efficient in preliminary tests. The efficiency of the NIR absorbers will investigated by monitoring the threshold of crosslinking. Effect of the light power will be investigated and also comparison between continuous and pulsed light.
2 laser configuration will be used here, all available at IS2M, some at NYCU :- 808 nm array high power VCSEL (60W) for continuous irradiation over large area (typ. 1 cm2). The interest of this laser source is its low-cost, which means that it can be applied to industrial applications, unlike other cost-effective laser (Deep-UV lasers, femtosecond lasers). This laser source will be used with shadow masks.
- Nd-YAG pulsed (Q-Smart Quantel), 6 ns, 450 mJ, 3 mm diameter, 1064 nm. This laser source will be used i) for homogeneous irradiation with beam extender, ii) with shadow masks for low resolution patterning, iii) focused beam for laser direct write and iv) phase masks for interference lithography. One interest of Nd-YAG is that they are already widely used in industry.
Moreover, pulsed irradiation is known to present specificity in thermal coupling as compared to continuous irradiation, so we expect specificities. However, NIR pulsed and continuous irradiation were never compared for photopatterning.
Our interest is not only to define the most effective light source to generate the desired effect on the material but, above all, study the light-matter coupling by systematic analysis of the material modification upon laser irradiation. FTIR, Raman, XPS, spectroscopic ellipsometry, XRD, TEM, etc… will be used to investigate these fundamental aspects. Direct temperature monitoring can be achieved
by thermal camera to characterize the final temperature obtained.
Note that this work has been started in the frame of our common ANR-MOST PRCI project (PhD of Ching-Fu Lin and Laurent Noel).
- Exploiting thermoplasmonic effects.
In this sub-topic, we propose to use the plasmonic properties of Au nanoparticles (NPs) as photoheaters for curing the sol-gel layer. This approach shows 2 main advantages: First, the extinction spectrum of the Au NPs can be tuned in a wide range of wavelength from visible to NIR, which give flexibility on the light source. Secondly, and more important, the final material is composed of Au NPs included in
semi-conductor matrix. In preliminary experiments, we showed that such hybrid metal/semi-conductor material exhibit interesting photoresistive properties that can be used in the design of photodetector. The spectral range of sensibility corresponds to the extinction spectra of the Au NPs, which means the PDs
can exhibit a wide range of photosensitivity (from UV to NIR).
For the Au NPs, we will investigate 2 different approaches that we have partially validated :- Au NPs prepared by colloidal synthesis. The interest of this approach is to provide nanoobjects with low polydispersity, that corresponds to narrow extinction spectrum. The Au NPs can be mixed in the sol-gel solution before preparing the thin film. This is very interesting to generate PDs sensitive at a well-defined wavelength.
- Au NPs arrays prepared by dewetting of Au thin films obtained by sputtering. Films with thickness in the range of 4 to 8 nm can be used to generate Au nanoislands by NIR laser irradiation. Using this property, it is possible to use NIR laser irradiation to generate the AuNPs in the first step and then cure the sol-gel layer in a second step.
The photoresponse of the PDs in various conditions will be carefully investigated to propose the mechanism of coupling between Au NPs and the semi-conductor, which appears as largely unknown at this stage.
- Carbon-based nanocharges.
The third route we like to explore if the doping of the sol-gel matrix with nanocarbon species such as carbon nanotubes (CNT) of graphen sheets (GS). In this case, the carbon nanocharges can be included directly in the sol-gel solution. Like the Au NPs, the CNTs and GS will have a double role: 1/ they will serve as strong NIR absorbers to generate strong photothermal effect. 2/ trapped in semi-conducting matrix, they can offer electron conduction channels that will improve the conductivity. In complement to the doping route, we will further investigate a route that we demonstrated recently (Yu et al, Adv. Mater. 2018) that consist is generating carbon species by high fluence pulse laser irradiation under oxygen-free conditions. Indeed, we proved in this first study that organic molecules used to prepare the metal oxide precursors can serve as C sources for generating in situ conducting C charges that improves a lot the conduction. - Application in wearable sensors
NIR laser annealing technology can achieve localized annealing, this is particularly important for developing wearable sensors on plastic or printed substrates. Sensors can be formed on various light, thin, and curved substrates and directly integrated with external circuits by using suitable 3D printed structure. The team in Taiwan has now begun to cooperate with medical doctors in neuroscience to develop fine motion detection, and the team in France has also begun to develop key technologies for making sensors on various sports equipment. The two teams also already successfully demonstrated an ultra-soft breath pattern detector on plastic wrap by using NIR laser annealed SnO2 device. This soft breath sensor can be easily integrated with a face mask to detect breath pattern, which is critical in monitoring diseases such as Snoring and Sleep Apnea Syndrome. The preliminary clinical trials have been examined by the medical doctors in Linkou Chang Gung Memorial Hospital in Taiwan. Further expansion of the clinical trials is under arranged. The NIR laser direct writing/annealing technology of this Taiwan-French cooperation will be able to show diversified sensor designs and applications in the future.
Figure 4. Different fields of application of metal oxides sensors that we will prepare by laser curing. These examples are all preliminary works developed in common between Prof. Zan and Dr. Soppera. This project aims at going further in the understanding of the process of such materials under laser,describe the sensing.
Figure 5. The ultra-soft NIR-annealed SnO2 humidity sensor on food plastic wrap to detect the breath pattern and to integrate with face mask.