Research
Theme 3
Excitonic Systems for Security, Lighting and Sensing
Manipulation, detection and use of light through excitonic materials are key concepts that will enable a raft of future technologies.
The overarching mission of this theme is to design and create materials with properties specifically defined for target applications that are relevant to industry. The Centre has worked closely with our core partners in this theme – DSTG (Platform 3.1), RBA (Platform 3.2) and CSIRO (Platform 3.3) – and has achieved significant targets and milestones including: development of new materials for chemical sensing, fabrication of blue-emitting quantum dot light-emitting diodes, while two new materials for document securitisation have been synthesised.
A Grand Challenge goal in this theme is to build an electrically pumped organic polariton laser. Such a low threshold laser has the potential to transform commercial applications of lasers.
Excitonic Sensors (Platform 3.1)
The goal of this platform is to develop emerging emissive materials, robust strategies and analysis procedures for miniaturising and simplifying luminescence chemical sensors with lower economic cost than existing large laboratory instruments, higher sensitivity than ordinary paper tests and better selectivity than electrochemical detectors.
In partnership with the Defence Science and Technology Group (DSTG), the Centre is in a unique position to develop portable and robust chemical sensors with high sensitivity and specificity.
DSTG is investigating technologies that can be included in such a device, with a particular focus on a photoluminescence-based response system.
We have developed a new device that allows the use of solution chemistry for chemical detection, greatly expanding the variety of probe materials that can be used. In addition to the detection of chemical species, an extension of the project has been devised to detect biological species.
Progress in 2022
A spectral down-shifting film was fully characterised and device integration experiments were performed. After a series of experiments, it was clear that the film made the excitation light more diffuse, and the optical output was significantly reduced. Further progress on this work package will require input from light emitting device experts with knowledge in light out-coupling.
Solution testing of a perovskite sensor for halides has been completed, including testing with sulfur mustard (HD). The sensor material can detect HD below one parts per million volume. Due to complex reactions and kinetics, quantification of the amount of HD with the current sensor system will be difficult.
An aerosol chemical sensor has been optimised to capture and detect fluorescein aerosol. Work has also been completed on the capture and detection of non-fluorescent chemical aerosols.
Research Highlights
The platform has been supporting the development of an aerosol sensor with microfluidic delivery system integrated. In collaboration with DSTG and a biomedical engineering group at the University of Melbourne, the integrated microfluidic system allows for solutions to be delivered onto the sensor substrate after aerosol has been captured. The primary target for this work is for the detection of SARS-CoV-2 using loop-mediated isothermal amplification (LAMP) assay.
The LAMP assay has been optimized allowing it to be conducted on a filter membrane suitable for the aerosol sensor device. This work also showed the reaction could be monitored with a modified version of the aerosol sensor optics.
Issues
Several projects concluded in 2022 with a contraction of personnel working in the platform. One of the challenges moving towards the end of Centre funding in 2024 is the recruitment of new students to work on the platform. We are addressing this by seeking support from DSTG which will enable full PhD scholarship positions until 2025-26.
Collaboration
Industry: Chemical detection team at DSTG
Outlook for next year
We aim to complete the development of the fluorescein detector and finalise the application of the aerosol chemical sensor for chemical systems of interest.
It is hoped that a fast and compact chemical sensor will be developed and ready to be deployed by the end of 2024.
In partnership with DSTG, an aerosol sensor has been developed. It has been shown to detect a fluorescent analyte, fluorescein, as well as some non-fluorescent analytes via in-situ chemical reactions. The device can be further developed into a compact package for a variety of mobile sensing applications.
Novel Security Features for RBA (Platform 3.2)
Australia leads the way in the development of polymer banknote technology. With counterfeiters gaining improved access to lower cost technologies, more sophisticated forgeries are possible.
To maintain confidence in the currency, development of new security features is needed. The most important security features that act as a first line of defence in identifying counterfeits are overt features. These features must be straightforward and intuitive for the public or cash handler to use in helping them identify the banknote is genuine. The security features must be durable, printable, difficult to replicate, and cost-effective.
Platform 3.2 is focused on the development of new overt optical security features for Australian polymer banknotes. The overarching goal is to produce banknote security features that are difficult to counterfeit and simple to verify. Such security features must be able to meet multiple requirements such as cost effectiveness, efficacy, durability, printability, and public safety.
Progress in 2022
The magnetic nanoparticles ink (MNI) project is aimed at developing an overt security ink using magnetically aligned nanoparticles to produce bright optical effects. Magnetic nanoparticles have been successfully synthesised, and the potential scaling up of such processes required to meet demand of high-volume banknote production has been explored.
While progress has been made in dispersing the nanoparticles into security screen printing inks, further work is required to obtain the desired optical effects. Effort has also been placed into modelling the potential optical effects of aligned magnetic nanoparticles, and methods for producing interesting optical effects via magnetic field manipulation.
The machine-readable near infrared inks project has progressed well during 2022, where a small amount of production material printed in 2021 demonstrated very promising results in both quality assurance/durability testing, and in the field with industry banknote processors.
Significant effort is being made in preparing the machine-readable NIR ink for a capability trial of approximately one million banknotes to fully assess the performance of the ink, in terms of printability, durability, and machine transport and functionality.
Research Highlights
Research highlights include:
- progress developing the synthesis processes of magnetic nanoparticles, and demonstrating impacts of scaling the process in the lab;
- Development of processes for dispersing nanoparticles into commercial security inks;
- Visible colour effects observed in cured commercial security inks on black background;
- Highly successful durability and industry testing of NIR inks.
Issues
The primary risks associated with the projects are:
- New materials may not be scalable, too costly for use in banknote technologies, or unable to generate the desired durability, efficacy or security for the project to progress;
- The RBA may change research directions in response to changing national and international demands on currency protection and/or actual or perceived counterfeiting risks;
- Inability to secure staff for key projects due to lack of domestic students qualified for the project or visa delays for talented overseas staff due to COVID restrictions.
Collaboration
Industry: Reserve Bank of Australia
Outlook for next year
Our goals for 2023 include a capability-scale print trial of the NIR ink, producing approximately one million banknotes. We aim to produce an MNI prototype sample set for initial durability and adversarial assessment. And we anticipate completing planning and preparation for a small-scale print trial of MNI features on production equipment.
Light-Emitting Devices (Platform 3.3)
This platform aims to deliver solutions for future lighting and display technologies by developing materials and devices beyond current efficiency, brightness and stability limits with spectral coverage from the ultraviolet to visible and infrared range.
These next-generation light-emitting devices (LEDs) will open new architectures and applications, such as tunable lasers.
We employ a combined theoretical and experimental approach towards the realisation of a stable blue LED, which has been challenging traditionally due to triplet losses and defect emission, and an electrically pumped polariton laser that remains one of the ‘Holy Grails’ in optoelectronics.
Progress in 2022
In 2022, we primarily focussed on fabricating and optimising various grating structures that can be later integrated with cladded structures in organic field effect transistors and are capable of outcoupling confined light.
We started new projects to exploit strong exciton-photon coupling in various photophysical processes, such as energy transfer and charge transfer dynamics in organic solar cells and intersystem crossing in thermally activated delayed fluorescence (TADF) molecules.
We also leveraged several different areas of research being pursued in the Centre on polariton devices and photophysics, polariton-assisted chemistry and plasmons-excitons, and contributed to a Chem Rev thematic review article on molecular energy transfer under the strong light–matter interaction regime.
Research Highlights
Active waveguide Bragg lasers via conformal contact PDMS stamps was published in Nature’s Scientific Reports. We fabricated active waveguide Bragg lasers via conformal contact between patterned PDMS stamp with an active organic semiconductor spin-coated thin film. In this configuration, feedback reflections are provided by PDMS-air gratings and the resonator can be detached from the active film. PDMS stamp can be removed and relocated to recover lasing post degradation without causing much change in lasing characteristics. The proposed device architecture is expected to accelerate screening of suitable lasing materials without increased fabrication cost.
Issues/risk mitigation
Efforts towards the development of QD-LEDs have been negatively impacted by disruption to student recruitment processes caused by COVID-19.
We have continued to explore a new opportunity which leverages from the expertise of other researchers in the Centre, namely the Bach Group’s world-leading back-contact electrode design, to develop a novel LED architecture. We will also examine graphene as a replacement for ITO in devices.
While having demonstrated optically pumped polariton lasing and explored different light-emitting device architectures, there is a risk that our eventual goals regarding electrically injected lasers cannot be achieved by the end of 2024 due to increased costs and lack of critical mass in terms of resources. A potential solution would be to refocus our attention from electrically injected lasing to cavity device physics and polariton photophysics.
Collaboration
International: Prof Satish Patil (IISc Bangalore).
Outlook for next year
By the end of 2023, we hope to extend the impact of strong light-matter coupling via polaritons beyond low threshold lasing to reveal new material photophysics. To this end, we aim to controllably manipulate strong-light matter coupling to enhance energy and charge transfer dynamics in organic solar cells, influence the rate of intersystem crossing and singlet fission in TADF and acene dimer systems, respectively, and lastly, use 2D nanostructures to enhance optical response for information processing.
We hope to achieve the successful fabrication of stable blue LEDs based on quantum dots, with full characterisation of their lifetime and spectral properties. This will be allowed by an in-house developed ligand exchange method and in-depth analysis of the electrical properties of LEDs.