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Plastic nanoscale electronics via novel fabrication paradigms

Periodic Reporting for period 1 - A-LITHIA (Plastic nanoscale electronics via novel fabrication paradigms)

Reporting period: 2016-04-01 to 2018-03-31

Nanogap electrodes are defined as two metals lying on the same plane and separated by an ultra-short channel of about 10 nm in length. Filling the nanogap with a functional material gives rise to electronic nanodevices and circuit elements with lower power consumption, faster speed, and higher level of integration than devices in vertical (sandwich-like) geometry.
Examples of devices benefiting from this architecture are high speed Schottky diodes implemented in radio frequency identification (RFID) tags and near-field communication (NFC) applications, enabling long-distance wireless communication among electronic devices, forming thus the cornerstone of what is nowadays called the Internet of Things. The large aspect ratio (width vs length) of these nanochannels may also be advantageous to nanoscale photodetectors, where the short nanochannel favours high sensitivity and fast response speed, characteristics well-sought after in light-sensing optoelectronic devices. Ability to fabricate these devices on plastic (flexible) substrates opens up further the application landscape to new opportunities, for example in the field of light-weight wearable electronics and low-cost smart labels.
The commonly employed fabrication techniques of nanogap electrodes are electron beam lithography or scanning probe lithography techniques, both of which are time-consuming and require expensive infrastructure. Therefore, the lack of a facile, inexpensive, high-throughput technique for the manufacturing of -especially dissimilar- nanogap electrodes, also on flexible substrates, has hindered their commercial and scientific exploitation.
The objectives of this project were, first, to optimise and further advance the process steps of an innovative, simple, low-cost and reliable technique, named adhesion lithography (a-Lith), which has been developed at Imperial College London, for the fabrication of symmetric or asymmetric metal nanogap structures on rigid and plastic substrates. Then, the aim was to fabricate various electronic and optoelectronic devices with these nanogap electrodes and characterise their performance. Finally, the possibility for industrial uptake of the technology would be also explored.
In the duration of the project, major steps towards upscaling the a-Lith process were performed and a range of electronic (radiofrequency diodes, resistive switching memories, molecular junctions) and optoelectronic (light-emitting diodes, photodetectors) devices were fabricated using the a-Lith fabricated nanogap electrodes on rigid as well as flexible substrates. The obtained results attracted significant industrial interest and further upscaling of the manufacturing process is now being undertaken in collaboration with an industrial partner, active in low-cost flexible electronics.
In the “A-LITHIA” project significant progress was made towards high throughput and high yield manufacturing of nanogap separated metal electrodes on different substrates using adhesion lithography (a-Lith). Optimisation of each process step and use of a semi-automated peeling system allowed control over the fracture mechanics and enabled the high-throughput fabrication of coplanar metal electrodes, consistently separated by a nanogap <15 nm. These electrodes were then implemented in the fabrication of several opto/electronic devices.
Radiofrequency (RF) rectifying Schottky n-type diodes fabricated with nanogap separated coplanar asymmetric Au/Al electrodes and ZnO-based films were proven to be particularly attractive devices, as ultra-high performance, reaching record cutoff frequencies (>>10 GHz) and increased output voltage was achieved. Furthermore, high performing p-type diodes operating at 13.56 MHz - a frequency of high commercial value, as many current applications involving contactless payment operate at this frequency - were also demonstrated.
Nanoscale optoelectronic devices were also investigated. A range of light-emitting polymers were used to fabricate nano-OLEDs spanning the whole visible spectrum. Optimisation of these devices was performed via selection of suitable interdigitated geometries of the nanogap electrodes and targeted surface treatment of the electrodes to tune their work function and achieve reduced turn-on voltages. Nanoscale UV-, visible- and NIR- sensitive photodetectors based on different types of materials were also demonstrated. These devices showed high responsivities and sensitivities and in some cases also fast photoresponse in the range of 100s of nsec and low operating voltages.
The phenomenon of resistive switching for application in non-volatile memory devices was thoroughly investigated, initially on the empty nanogap electrodes and then by depositing different types of organic and inorganic semiconducting materials on the nanogap electrodes. The devices demonstrated multilevel switching and high endurance using coplanar nanogap Al/semiconductor/Al capacitor structures. Some preliminary results on molecular junctions fabricated with asymmetric (Al/Au) electrode nanogap structures were also obtained.
The research outcomes are summarised in the three published peer-reviewed articles, while four more articles are currently being drafted. Furthermore, fruitful collaboration with the secondment host (TNO/Holst Centre) resulted in compilation of a topical review article on “Flexible Diodes for Radio Frequency (RF) Electronics: A Materials Perspective”, which helped in mapping the current landscape of flexible RF electronics and define the state-of-art materials and techniques used in this field of high commercial interest.
Dissemination of the research performed during the project was also realised through a number of oral and poster presentations at international scientific conferences. Participation in multiple outreach activities communicated the research to various audiences ranging from general public to investors, entrepreneurs and politicians.
Finally, exploitation of the potential for further industrial uptake of the a-Lith technology after the end of the fellowship was obtained via (a) organisation of an industrial workshop for partner identification and (b) award of a knowledge transfer secondment grant together with a UK SME, which is a leader in the field of ultra-low cost flexible electronics.
The nanoscale opto/electronic devices that were fabricated within this project bear the potential for exploitation in a plethora of applications including -but not limited to- displays, RFID tags/smart labels, photodetection, energy harvesting systems and information storage. This research is, moreover, directly relevant to the field of plastic electronics, rendering it absolutely timely due to the great interest, worldwide, in commercialising such products. For instance, according to the IDTechEx market analysis report, the total market for printed, flexible and organic electronics will grow from $29.80 billion in 2015 to $73.69 billion in 2025. The majority of that is OLEDs, whereas stretchable electronics, logic and memory, thin film sensors are much smaller segments but with huge growth potential as they emerge from R&D. Therefore, the outcomes of this project are contributing significantly in the technological progress and the fast commercialisation of such devices that will pave the way to the electronic devices and appliances of the future, bringing the society one step closer towards the Internet of Things era.
Electronic devices fabricated with nanogap coplanar electrodes