Research
Theme 2
Control of Excitons
Realising control over excitons to build new excitonic devices and technology.
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 2022
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 candidate molecules have been identified for multiphoton (simultaneous and sequential two-colour) exciton logic.
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
We have been developing a modular computational pipeline to model singlet fission in dimers. Starting from a structure, we use DFT to determine potential energy surfaces, along with associated exchange interactions. A Monte-Carlo approach is used to model structural dynamics, providing a time dependent exchange interaction between chromophores. Spin dynamics are obtained which qualitatively match experiment. While at this point we have demonstrated what is effectively a minimum viable product, there is significant potential for improvement at each step by research groups focused on these areas.
Issues/risk mitigation
As we approach the end of the Centre, staffing will become a more significant risk. Depending on the timing of staff departures, reappointment of staff will become difficult, as will attracting new staff if only short-term contracts can be offered. The broader challenges around recruiting, visas and international mobility will exacerbate these issues.
An ongoing risk relates to the failure of the relevant lasers and other hardware, however the systems are well-maintained (some under warranty until mid to late 2023), which mitigates this risk.
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 hope to achieve a working exciton logic gate by the end of 2023, followed by a series of working exciton logic gates in 2024.
The major priority to enable this goal to be achieved is the recruitment of more graduate students.
We aim to establish a mature field of exciton logic as a legacy of this Centre’s platform.
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 large number of nanocrystal assembly methods that allow different types of nanoparticles to be ordered and co-ordered into structures with a wide variety of geometries.
Progress in 2022
We have demonstrated control of exciton diffusion in single multichromophore polymer chains and films.
A numerical method for modelling groups of two or more quantum-dots, including their magnetic field response and optical emission characteristics, has been developed.
We have developed a full theoretical model for optical and phononic response in fluorophore- localized surface plasmon resonance (LSPR) systems, specifically nitrogen vacancy (NV)-LSPR systems. This shows metal nanoparticles can be utilised to control the optical emission (both rate and intensity) of NV centres.
A rational tool has been developed to understand and predict the optical properties of colloidal Zinc selenide (ZnSe) nanoplate systems.
A theory has been developed to explain the limits of electrophoretic deposition as a method to assemble discrete nanoparticle arrays.
We have also achieved a new understanding of the 2D perovskite film structure.
Research Highlights
Zhang, H., Liu, Y., Ashokan, A., Gao, C., Dong, Y., Kinnear, C., Kirkwood, N., Zaman, S., Maasoumi, F., James, T. D., Widmer-Cooper, A., Roberts, A., Mulvaney, P., A General Method for Direct Assembly of Single Nanocrystals. Adv. Optical Mater. 2022, 10, 2200179. https://doi.org/10.1002/adom.202200179
Nisar, A.; Hapuarachchi, H.; Lermusiaux, L.; Cole, J. H.; Funston, A. M. Controlling Photoluminescence for Optoelectronic Applications via Precision Fabrication of Quantum Dot/Au Nanoparticle Hybrid Assemblies. ACS Applied Nano Materials 2022, 5 (3), 3213 - 3228 DOI: 10.1021/acsanm.1c03522.
Hapuarachchi, Harini, Campaioli, Francesco and Cole, Jared H.. "NV-plasmonics: modifying optical emission of an NV− center via plasmonic metal nanoparticles" Nanophotonics, vol. 11, no. 21, 2022, pp. 4919-4927. https://doi.org/10.1515/nanoph-2022-0429
Issues/risk mitigation
Progress in this platform was slower than expected throughout the COVID pandemic. The effects of the lockdown have lessened in 2022.
However, we have noted a break in the usual pipeline of PhD students due to difficulties recruiting students internationally, as well as difficulty hiring postdoctoral researchers internationally.
The latter issue has been offset by the availability of Centre-wide postdoctoral schemes.
Collaboration
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), successful submission of IRTG application (Bayreuth).
Outlook for next year
A key enabling step will be the complete planning of concept, and building of a team, to work on a sophisticated nanostructured device to achieve optical realisation of a NAND gate using two coupled QDs.
Beyond this, we aim to construct one, or more, functional super-nanostructures. To achieve this, additional PhD students/postdocs need to be recruited.
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 2022
Over the last year, a series of papers have appeared either demonstrating the techniques we have developed or using the techniques in collaborative Centre papers.
Several new software packages have been released for public access and/or published in computational modelling journals.
During this year, nine Research Fellows worked on projects within this platform, as well as at least seven students whose work depends on the methods being developed.
Research Highlights
Researchers at RMIT and Monash University have continued to excel in machine learning for materials discovery, including for perovskites.
Representative of these efforts is the paper ‘Machine Learning-Enhanced High-Throughput Fabrication and Optimization of Quasi-2D Ruddlesden-Popper Perovskite Solar Cells’, which has been submitted to Advanced Energy Materials.
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.
However, we are already seeing loss of knowledge as postdocs move on to new jobs. To help address this, it will be important to plan any remaining calculations that are needed to support future experiments.
Over the next 18 months, it will be critical to manage computational support for the other platforms in the Centre.
Collaboration
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 Chemistry, University of Sheffield, Sheffield, United Kingdom)
- Jochen Feldmann (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 2023 is to apply the methods we have developed to problems of interest in the Centre.
We aim to demonstrate multi-scale modelling of exciton transport from the ab-initio to the device level.
By the end of 2024, we aim to have several examples of multi-scale models validated against experiment.
This work requires close collaboration with the experimental platforms to ensure the required simulations and experiments are done in time to combine them and provide a quantitative comparison.