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Transparent and flexible electronics with embedded energy harvesting based on oxide nanowire devices

Periodic Reporting for period 4 - TREND (Transparent and flexible electronics with embedded energy harvesting based on oxide nanowire devices)

Reporting period: 2021-07-01 to 2022-09-30

The Internet of Things is shaping the evolution of information society, requiring an increasing number of objects with embedded electronics, sensors and connectivity. This spurs the need for systems where summing to performance and low cost, multifunctionality has to be assured. To date, there is not a technology fulfilling these needs: for instance, thin film microelectronics enables flexibility and large area processing but fails on assuring high levels of electrical performance or energy harvesting; nanoscale Si or III-V materials, while exhibiting outstanding electrical performance, typically fail on multifunctionality and/or low-temperature & large-area processing.
In this context, TREND aimed to take transparent electronics into as-of-yet unexplored levels of functionality, by combining on flexible substrates transparent and high-speed nanocircuits with energy harvesting capabilities, based on multicomponent metal oxide materials, particularly nanowires (NWs). For this end, sustainable and recyclable materials as zinc-tin oxide (ZTO) were synthesized using low-temperature and low-cost solution processes. The precise control of the density and alignment of these nanostructures is critical for device integration and in TREND different approaches were considered to tackle this, from transfer methods to direct growth of NWs from nm-scale patterned seed layers on flexible substrates. Major applications envisaged were flexible nanotransistors, that soon will enable comparable or even superior integration densities on flexible substrates compared to sub-22 nm nodes in Si technology, as well as piezo/triboelectric nanogenerators able to scavenge energy from human body movement, demonstrated within TREND on flexible polymeric substrates and on carbon fibers weaved on textiles. But other fascinating properties of these materials were also explored for a multifunctional platform, such as photocatalytic and sensing ones. With the developed nanoscale processing technologies, we could also demonstrate transparent and flexible electrocorticography electrode arrays in in-vivo neural recordings. On the circuit level, multiple analog/digital circuit blocks were demonstrated on flexible substrates, exploring circuit design techniques to compensate for oxide transistor performance limitations compared to Si CMOS and by establishing baseline processes enabling miniaturized vias through multilevel metallization schemes (up to 4 metal levels demonstrated), fully compatible with flexible and large area electronics tools.
Tweaking the optimized processes developed for transistors/circuits, particularly multilayer insulators, we could also verify unprecedented sensitivity levels on flexible oxide transistors used as ionizing radiation detectors, which triggered an ERC Proof of Concept Grant starting in October 2022 (FLETRAD, GA 101082283) to explore commercialization routes of such technology.
TREND was a very ambitious interdisciplinary project motivating advances in materials science, engineering, physics and chemistry, with impact extending from consumer electronics to health monitoring wearable devices. By promoting new ideas for practical ends, it contributed to place Europe in a leading position of flexible electronics, where sustainability of materials and processes tackled with multifunctional concepts are key factors.
An overview of the main results obtained throughout TREND is provided below:
- Zinc-tin oxide nanowire (ZTO NW) synthesis below 200 °C through an hydrothermal method, seed layer free, or alternatively, from ZnO nanowires conversion.
- p-n heterojunctions using solution processed nanostructures of zinc-tin oxide and copper oxide.
- Oxide thin film transparent conductors developed through solution combustion synthesis and sub-200 °C atomic layer deposition.
- Alternative transparent conductors developed using metallic nanowire spray coating and sub-μm patterned metal grids, both on flexible substrates, with easily tailorable optical and electrical properties by mesh design. Figures of merit (optical/electrical) exceeding those of competing approaches (e.g. graphene, CNTs, TCOs)
- Substrate conformable imprint lithography processes to achieve metal linewidths of 40 nm on flexible substrates.
- Hybrid nanogenerators making use of triboelectric and piezoelectric effects to scavenge energy, based on ZTO NWs/PDMS on flexible polymeric substrates or ZnO NWs/PDMS on carbon fibres, with instantaneous power densities above 200 µW.cm-2.
- Software platform for easy and fast empirical model extraction of oxide transistors, based on artificial neural networks. Also, physical-based model developed for sub-μm devices.
- Flexible ZTO TFTs with comparable performance to mainstream IGZO TFTs (µFE>5 cm2/Vs, Von~0 V, On/Off>1E7). Electrolyte-gated ZTO NW transistors on paper, with On/Off>1E4 Von~0 V and µFE>1 cm2/Vs. Establishment of fabrication processes to obtain self-aligned oxide TFTs with channel lengths down to 500 nm, with cut-off frequencies >600 MHz.
- Exploring circuit design techniques unusual in flexible electronics to tackle limitations of oxide transistor technology compared to conventional Si CMOS, such as switched bootstrapping capacitive loads in logic gates for rail-to-rail operation and level-shifting ability, negative capacitance generators to increase amplifiers bandwidth, fully-transistor based rectifiers operating at NFC or high-frequency range (13.56 MHz) of RFID.
- Establishment of processes to fabricate oxide electronics with large number of layers (>10) and enable miniaturized vias through multilevel metallization schemes.
- Application of multiple nanostructures within ZTO system for photocatalysis (degradation of organic pollutants), both with UV and visible light.
- Integration of miniaturized oxide TFTs with memristors on flexible active crossbar arrays, making use of all the deposition/lithography processes established during TREND, for new computation paradigms.
- Flexible ionizing radiation sensors based on oxide TFTs using multilayer dielectrics deposited by ALD, with sensitivity exceeding 30 V/Gy.

Results in TREND were extensively disseminated:
• 51 peer-reviewed publications (scientific journal articles, book and book chapters and PhD/MSc dissertations).
• 49 presentations in scientific conferences and workshops, including 3 key-notes. Additionally, invited contribution on ERC TREND concept for high-level EC delegates visiting Portugal.
• Dissemination to general audience was also a concern, with the team participating in 2 European Research Night events and 4 videos/interviews to radio and internet news channels. The team was also present in multiple events promoted on a yearly basis at FCT-NOVA where young students (basic and high-school level) have the possibility to know the research being done at campus.

Regarding exploitation:
• ERC PoC FLETRAD originated from ERC TREND to evaluate commercialization potential of ionizing radiation detectors based on oxide transistors.
• Strong contribution to setup portfolio of nanofabrication/nanocharacterization services currently being provided at CENIMAT|I3N research center for academia and industry. 1 start-up company already with a 18 month contract exploring this, besides multiple individual works for other costumers.
Zinc-tin oxide thin film transistors
Photocatalysis using ZTO nanostructures under UV and visible light
Mechanical energy harversters with ZTO NWs and PDMS
Transparent and flexible ECoG electrodes for in-vivo neural recordings
Single ZnSnO3 NW by hydrothermal synthesis at 200 °C - electrical characterization
Concept behing ERC TREND project
Zinc-tin oxide nanostructures