Control of Excitons

Through theory, advanced spectroscopic analysis and new materials, we are getting better understanding and control of the four dimensions of an exciton: Energy, Lifetime, Position and Spin/Polarisation. The outcomes of this theme include new ways to coherently control excitons, new excitonic materials, nanostructure controlled excitonic motifs and new theories of exciton transport.

Excitons are short-lived electron-hole bound pairs which are created when a light particle (photon) interacts with an atom, molecule, nanocrystal or polymer. The conversion of light into electrical energy (in solar cells) and electrical energy into light (LEDs) occurs via excitonic processes, and understanding their properties is essential to the development of new materials that have a higher light-to-electrical-energy conversion efficiency (and vice versa).

In Theme 2 we seek to understand the fundamental processes that govern exciton generation, lifetime and transport across different length scales. This theme comprises three research platforms: Coherent Control of Excitons (Platform 2.1), Excitons at Interfaces (Platform 2.2) and Multiscale models of exciton transport (Platform 2.3).

Coherent Control of Excitons (Platform 2.1)

What’s this platform about?

We are exploring how to control quantum excitonic phenomena, using a range of approaches, including manipulation of spin and optical polarisation.

This will provide control over how far, how quickly and to which location an exciton can migrate within a material, and how they interact with each other and their environment.

We seek to understand and control the process of singlet exciton fission, in which two excited states with triplet spin character are generated from a photoexcited state of higher energy with singlet spin character.

This process is a pathway to surpassing the Shockley–Queisser limit of photovoltaic cell efficiency, as it allows for more than one exciton to be generated by a single optical excitation.

Progress in 2023

A new laser system at the University of Melbourne is now configured for three or four different colour femtosecond pulses for timed pulse control of excitons, to study the fate of excitons and for exciton logic.

New directions for implementing exciton logic have been identified. This includes candidate molecules for multiphoton (simultaneous and sequential two-colour) exciton logic and three-colour devices.

Spatially resolved, magnetic field-dependent electroluminescence (EL), photoluminescence (PL) and electrically detected magnetic resonance (EDMR) in a variable temperature system is now available at UNSW.

We have also implemented:

• instrumentation to allow measurements from femtoseconds (fs) to milliseconds (ms) on one instrument with excellent signal-to-noise ratio;
• transient infrared spectroscopy over various timescales;
• magneto-cryo optical time-resolved spectroscopy.

Research Highlights

Issues/risk mitigation

As we approach the end of the Centre, we are losing valuable staff, so no new work will be initiated. Projects are being finalised and final publications are in preparation. There are already issues relating to how we get some projects finished off and published. (Both Jamie Laird and Rohan Hudson have gained jobs starting in January 2024 at CSIRO and MOGLabs respectively).

An ongoing risk relates to the failure of the relevant lasers and other hardware. As the Centre winds up, maintenance of the instrumentation established during the Centre will become more problematic.

Collaborations

International: Ron Steer (U Saskatoon, Canada) S2 emission from azulenes, xanthiones etc. (confidential), Daniel Congreve (Stanford, USA), Luis Campos (Columbia U, USA), Klaus Meerholz (Cologne, Germany).

Outlook for next year

We have demonstrated a simple example of an exciton logic AND gate based on timed femtosecond pulses exploiting two-photon, two-colour absorption to provide emission as the signal. Other operational gates (e.g. OR) can also be demonstrated. Three chromophore energy transfer-based logic will be demonstrated in 2024. We aimed to establish a mature field of exciton logic as a legacy of this Centre’s platform. The publication of the Nature Reviews Chemistry paper outlines the future of this new branch of research.

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Excitons at Interfaces (Platform 2.2)

What’s this platform about?

Exciton dynamics in materials and across interfaces are highly complex and are dependent on the nanoscale morphology, spin polarisation, exciton manifold and interfacial density of states.

Our aim is to understand how these parameters control the exciton dynamics in assemblies of organic-inorganic hybrid systems.

These parameters are not always observable by conventional optical methods and require super-resolution and single molecule fluorescence techniques to probe below the diffraction limit of light.

We have developed a suite of nanocrystal assembly methods that allow different types of nanoparticles to be ordered and co-ordered into structures with a wide variety of geometries and properties; and applied advanced characterisation tools and theory to explain exciton dynamics in these systems. We are now working to integrate these findings into functional nanostructures.

Progress in 2023

We have applied numerical techniques developed in Platform 2.3 to the study of optical emission from nanoplatelets (Yuan et al. ACS Nano 2023), the lifetime dependence of quantum rings (Sullivan & Cole, arXiv:2308.11843, under review) and the formation of charge transfer excitons at the silicon/tetracene interface (Klymenko et al. arXiv:2310.15736, under review).

In addition, our previously published model of how nitrogen-vacancy centers are modified by a nearby localised surface plasmon resonance has been generalised to include spin dependent effects. This has allowed us to predict enhanced signal to noise in magnetic resonance and quantum computing applications (Hapuarachchi et al. arXiv:2311.14266, under review).

Energy transfer between CdZnS quantum dots and perylene diimide dyes has been investigated, with maximum energy transfer efficiencies of 91% achieved from QDs to PDIs (Wu et al. J Phys Chem C 2023). These systems are promising models for artificial photosynthesis and down-converter systems for LEDs and displays.

Methods have been developed for modelling the conformational states of molecules on nanoparticles and applied to understanding the colloidal stability of nanoparticles in solvent mixtures (Hasan et al. ACS Nano 2023) as well as energy transfer between ATTO dyes relevant to interpreting super-resolution optical microscopy experiments.

Time-resolved emission and transient absorption microscopy setups have been developed for probing exciton behaviour in well-ordered multi-chromophore systems, and work has continued on probing photocurrent and exciton dynamics in thin films of metal halide perovskites.

We have characterised the spectroelectrochemistry of CdSe/CdxZn1-xS quantum wells (Ashokan et al. ACS Nano 2023) and made progress in controlling the assembly of nanoplatelets and electrophoretic deposition of nanoparticles, along the pathway towards building a functional nanostructured device.

Research Highlights

Issues/risk mitigation

Progress in this platform was slower than expected throughout the COVID pandemic.

The effects of the lockdown lessened in 2022-23 with the ability to recruit new PhD students and postdocs, partly supported by a Centre-wide postdoctoral scheme.

Looking forward, the main risk is in maintaining expertise and momentum as ARC funding for the Centre ends. Recent recruitment of PhD students through the IRTG project with Bayreuth and other funding sources is helping to mitigate this transition.

Collaborations

Industry: Anthony Chesman (CSIRO)

International: Prof. Xiaotao Hao (PI, Shandong University), Prof Koehler (PI, Bayreuth University), Forschungszentrum Jülich, Prof Colfen (Konstanz University), Prof Weber (University of Colorado Boulder, USA), Prof Rodriguez (Santander, Spain), Prof. Anindya Datta (IIT Bombay, India), Prof Tobias Kraus (Leibnitz Institute for New Materials, Germany), IRTG project (Bayreuth).

Outlook for next year

As the Centre draws to a close, the focus is on constructing functional super-nanostructures and making progress towards building a sophisticated nanostructured device to achieve optical realisation of a NAND gate using two coupled QDs.

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Multiscale models of exciton transport (Platform 2.3)

What’s this platform about?

Exciton dynamics in materials and across hybrid interfaces are highly complex and are dependent on nanoscale morphology, spin polarisation and vibrational properties.

The aim of this platform is to develop computational modelling techniques to understand exciton dynamics in single molecules or organic-inorganic hybrid systems, all the way up to the device level.

The central work of this platform is to develop methods and approaches which are then used to tackle the problems in the other platforms of the Centre.

Progress in 2023

Over the last year, a number of papers have been published demonstrating the Centre’s ability to apply machine learning and ab-initio methods to problems of interest in Exciton Science. These include Meftahi et al. Advanced Energy Materials 2023, Ye et al. Chem 2023, and Li et al. Solar 2023.

The Centre’s work on developing and applying open-quantum systems and other effective methods to Exciton Science problems has also seen several new publications including Collins et al. Communications Physics, Forecast et al. Journal of Chemical Theory and Computation 2023, and Yuan et al. ACS Nano 2023.

In general, the Centre has now demonstrated accurate simulations methods spanning all the way from the atomic to the device scale. This provides invaluable input into the design and analysis of experiments in Exciton Science and the development of novel light control technologies.

Research Highlights

Issues/risk mitigation

Much of our technique-specific knowledge rests with particular Research Fellows. As much as possible, this knowledge is being stored in papers, theses and technical notes.

In the final six months of the Centre, the majority of remaining results will be drafted and submitted for publication, as well as software packages archived or released on publicly available repositories.

Collaborations

Industry: HQS Quantum Simulations

International: Per Delsing (Department of Microtechnology and Nanoscience, Chalmers University of Technology, Gothenburg, Sweden), Liang Tan (Theory of Nanostructured Materials, Lawrence Berkeley National Lab, USA), Robert Shaw, (Department of hemistry, University of Sheffield, Sheffield, United Kingdom), Jochem Feldman (Physics & Nanosystems Initiative München (NIM), Ludwig Maximilians Universität, Munich, Germany), Yinyin Bao (Institute of Pharmaceutical Science, ETH Zurich, Zürich, Switzerland

Outlook for next year

Our focus in 2024 is publishing the remaining key outcomes from the Centre, including multiple influential results on exciton transport, photochemistry simulation and multi-scale models.

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Rapid Materials Discovery (Platform 2.4)

What’s this platform about?

A pioneering integration of robotics, automation, and machine learning, aiming to position Australia at the forefront of global materials discovery.

The Rapid Materials Discovery platform focusses on the new High-Throughput Materials Discovery facility being installed at the Melbourne Centre for Nanofabrication. This facility is a collaboration with hardware installation, operation and management led by Exciton CI Prof. Udo Bach and machine learning capabilities led by Exciton CI Salvy Russo, with funding and resources from ACAP, Exciton Science, Monash University and CSIRO.

The High-Throughput Materials Discovery facility represents a significant advancement in the field of material science. At its core, this unique facility is designed to blend state-of-the-art robotics, automation, and machine learning technologies to create a user-centric research environment. It features a 13-meter long, fully inert robotic platform situated in a cleanroom environment. This advanced setup is capable of automatically fabricating and testing up to 2,000 new material samples every week – several orders of magnitude faster than human researchers, and with far no human-induced variations and errors.

Initially, the platform's research efforts will be centred around solar cell materials, with a particular emphasis on perovskites and other emerging. However, its scope is not limited to this field alone and the facility is also expected to significantly contribute to other materials research fields such as solar fuels and battery materials.

The platform is housed within the Melbourne Centre for Nanofabrication (MCN), strategically located adjacent to Monash University and the Australian Synchrotron. This location not only provides the necessary infrastructural support but also fosters a collaborative environment that is conducive to groundbreaking research and innovation.

By combining these elements, the High-Throughput materials discovery platform is set to make Australia one of the world's leading destinations for the discovery and testing of new materials.

Progress in 2023

Significant milestones reached: proof-of-principle publication, off-site system testing in Switzerland, and installation at the MCN, paving the way for future discoveries.

The year 2023 marked a period of substantial advancement for the High-Throughput materials discovery platform. A significant highlight was the publication detailing the platform's proof-of-principle work led by Dr. Nastaran Meftahi at RMIT and Dr. Adam Surmiak at Monash University, which successfully demonstrated the synergy between fully robotic solar cell fabrication and machine learning optimization. The innovative approach taken by the Exciton Science team led to the discovery of a new perovskite material composition that surpasses the performance of samples optimised by human-led methods.

The installation of the High Throughput Materials Discovery facility is also nearing completion. Dr. Adam Surmiak played a crucial role in this phase, dedicating several months to work alongside robotics supplier ChemSpeed in Switzerland. His commitment was directed towards testing, debugging, and optimizing the system, ensuring its readiness and efficiency before its shipment to Australia. This hands-on approach was pivotal in ironing out any potential issues prior to shipping.

Upon the completion of these preparatory stages, the full system has been shipped and successfully installed at the Melbourne Centre for Nanofabrication (MCN) in Clayton, Victoria. All four glove boxes integral to the system's operation have been put in place, marking a significant milestone in the setup of this sophisticated platform.

The final steps of assembling and testing the robotic synthesis, handling and characterisation tools within the gloveboxes are currently underway, signalling the nearing completion of the installation of this ambitious facility. The progress made in 2023 has laid a solid foundation for the platform's operation and future achievements.

On the Machine Learning side, we published a combinatorial, machine learning (ML) enhanced high-throughput perovskite film fabrication and optimization study. This work designed a bespoke experimental strategy and produced perovskite films with a range of different compositions using only spin-coating free, reproducible robotic fabrication processes. The performance and characterization data of the solar cells produced were used to train a ML model that allow materials parameters to be optimized and direct the design of improved materials. The new, ML-optimized, drop-cast quasi-2D RP perovskite films yielded solar cells with power conversion efficiencies of up to 16.9%. This work was published in the Journal Advanced Energy Materials (Adv. Energy Mater, (2023,)13, 2203859).

Research Highlights

Issues/risk mitigation

Due to the bespoke nature of the high-throughput materials characterisation system, and reliance on external companies to build, test and ship the system, this platform has taken longer than expected to be installed in Australia. However, the system is now in-place at the Melbourne Centre for Nanofabrication. Hiring is underway for another engineer with programming and/or robotics experience to ensure that the facility can quickly transition from installation phase to testing and full use.

Collaborations

Industry: Australian National Fabrication Facility (ANFF), Melbourne Centre for Nanofabrication (MCN), CSIRO, ChemSpeed

International: State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, China

Outlook for next year

The High-Throughput Materials Discovery facility is set to become a major part of Exciton Science’s legacy.

Once installation and testing have been finalised in mid-2024, this platform is expected to ensure Australia’s place at the forefront of material discovery, providing unprecedented speed and precision in the fabrication and testing of new materials.

Full operation is anticipated in mid to late 2024 with and first publications fully utilizing this system will follow in late 2024 to early 2025.