Platform Leader: Dane McCamey
Deputy Platform Leader: Trevor Smith
To exploit excitons fully for advanced applications in areas such as optical molecular switching, sensitive magnetometry, and spin-based logic, control over the quantum degrees of freedom (such as phase, super-position and spin) must be established.
In this platform, we are exploring how to control quantum excitonic phenomena, using a range of approaches, including manipulation of spin and optical polarisation, and with tailored pulse sequences. 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.
In particular, we seek to understand and control the process of singlet exciton fission, in which two excited states with triplet spin character (triplet excitons) are generated from a photoexcited state of higher energy with singlet spin character (a singlet exciton). 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.
A recent highlight from this platform is a collaborative effort with colleagues at Columbia University and CUNY, where it was shown how one can decouple the generation rate of pairs of excitons with correlated spin from their lifetime, which is important for applications in solar energy harvesting.1
Exciton Science Research Groups
Provides expertise related to coherent control of the spin degree of freedom of excitons, as well as exciton device fabrication. Collaborates with the Schmidt group on multi-exciton processes, the Wong group on magneto-TTA, the Mulvaney group on nanoscale devices, and the Cole group on the theory of multi-exciton processes.
Provides expertise in theory and modelling the excitonic coupling within bichromophoric molecules, of which bilirubin is an example. This work will be extended to more complex systems in the near future. The Cole group collaborates broadly with the McCamey, Schmidt, Smith and Wong groups.
Steady-state and time-resolved spectroscopy of the excitonic behaviour and dynamics in bi- and multichromophoric molecules. Multiple ultrafast laser pulses of different wavelengths can be timed to arrive in various sequences.
Singlet fission in dimers, polymers and nanoparticles.
Provides expertise in theory and modelling.
Luis Campos (Columbia University, USA)
Matthew Sfeir (Brookhaven National Lab/CUNY, USA)
R.P. Steer (University of Saskatoon, Canada)
The Coherent Control of Excitons platform had a productive 2019, with over 20 publications in leading journals. We investigated the spin coherence properties of excitons in a range of materials and processes, including multiexciton processes such as singlet fission and TTA upconversion. In order to understand these properties fully we have developed and published a theoretical framework for predicting the populations of quintet states which arise during the singlet fission process, based on temporal variation of the exchange interaction between coupled molecules, particularly dimers.
Substantial work has gone into investigating the influence of spin and magnetic fields on exciton lifetime, with the aim of developing technologically relevant films of materials which support singlet fission. Figure 1 shows measurements of films of acene dimers which allow us to separate the rate of fission from their lifetimes. Additional work to understand the variation of spin properties in monolithic devices is also underway at UNSW.
During 2019 we raised $200k from the UNSW Research Infrastructure Scheme, which will be used to purchase a cryo-free magnetooptical measurement platform. This will be available for use by the end of 2020. This equipment will enable us to investigate exciton diffusion and coherence via optical probes. This will be installed in new laboratory space adjacent to the UNSW Laser Laboratory and electron spin resonance instrumentation.
Bichromophoric molecules allow modelling and measuring of excitonic coupling between the two chromophores, and can form the basis of understanding exciton behaviour in more complex systems. An example of this is bilirubin, an important metabolic molecule. We have investigated the exciton coupling in a series of bilirubin analogues that we have specifically designed and synthesised.
We also aim to control the fate of excitons by optically interfering with the energetics of the system through which the exciton migrates. We have initiated a program of synthesis and characterisation of multi-chromophore systems to investigate the use of timed femtosecond pulses to control the fate of excitons, along the lines outlined in the schematic. The base unit is a photochromic switch to which other chromophores are attached (Figure 2). A pulse of one wavelength excites the donor (e.g. tetracene), a pulse of a different wavelength can induce the photochromophoric switch and a 3rd chromophore (e.g. pentacene or a red fluorescent dye) can be photoexcited with a pulse of a 3rd wavelength. Inducing the photochromic switch results in a change in molecular structure and distance between chromophores and by timing the arrival of these pulses of different wavelengths the transfer of excitonic energy can be modulated.
We also aim to optically control the transport of excitons from the point of formation to other points in an assembly (on a micro-macro spatial scale). In order to rapidly characterise excitonic processes on these scales we are developing a series of optical microscopy techniques with high spatial and temporal resolution. We have established protocols for using light to control the relocation of halide ions (and thereby the light emitting properties) in mixed halide perovskites. We have developed microscopy methods to induce and monitor these changes in real time on single crystals of micrometre dimensions.2 We are also developing transient absorption microscopy capabilities to allow us to probe ultrafast dynamics on sub-micron scales, in addition to time-resolved super-resolution optical microscopy methods (structured illumination and stimulated depletion microscopies).
Additional ultrafast laser capabilities for the Centre will be enabled in 2020 through the award of an ARC Linkage Infrastructure and Equipment Funds grant. This will provide a broader range of wavelengths, narrower and higher energy pulses than currently available to Exciton Science, and new modes of detection (such as femtosecond infrared spectroscopy and multiple pulse coherence measurements).
Finally, we are using MRCI/DFT & TD-DFT methods to model the singlet and triplet excitation energies of conjugated molecules. Polyacenes are used as case studies. Theoretical work is underway at the RMIT node, with experimental development at UNSW aimed at developing robust methods to measure triplet energies via photothermal techniques to validate these measurements.
1. Pun AB al., ‘Ultra-fast intramolecular singlet fission to persistent multiexcitons by molecular design’, Nature Chemistry, vol. 11, pp. 821 – 828.
2. W. Mao, C.R. Hall, A.S.R. Chesman, C. Forsyth, Y.-B. Cheng, N.W. Duffy, T.A. Smith, and U. Bach, “Visualizing Phase-Segregation in Mixed Halide Perovskite Single Crystals”, Angew. Chem. Int. Ed., 58, 2893 –2898 (2019) doi:10.1002/anie.201900853
An overarching aim of this platform is the production of an exciton-based logic device. To achieve this aim requires a thorough understanding of the spin coherence and spatial characterisation of excitons. This demands the development of advanced spectroscopy techniques supported by theoretical and computational modelling. New techniques including time-resolved electron-spin resonance spectroscopy, advanced optical microscopies and ultrafast laser spectroscopy techniques are being developed to this end. The theory and modelling efforts involve the development of MRCI/DFT and TD-DFT methods for describing processes such as singlet fission. We are currently seeking efficient ways of reducing the computational complexity of the second-order tensor operator and build a model that will help in engineering novel TTA and SF material.
In 2020 we are aiming to deliver a concept for an exciton logic device which may exploit either the spin or charge degrees of freedom (or both) alongside active control of molecular structure. We will also continue to understand exciton coherence and lifetime in a range of materials and systems, as controlling these is critical to producing high fidelity logic devices irrespective of the preferred implementation which is identified.
We will install a new femtosecond laser system at the University of Melbourne that will enable several new modes of spectroscopy and the multiple pulse techniques we have proposed, and will also expand the low-temperature microscopy and spectroscopy capabilities at UNSW Sydney.
We will continue to work with collaborators both in Australia and internationally to source and develop molecules which support the aims of this program, including novel dimers and photoswitches.
Identifying and acquiring suitable multichromophoric molecules has been problematic but enlisting additional assistance in the synthesis of novel photoswitching molecules has alleviated some of these challenges.
New facilities will enable multiple pulse femtosecond laser experiments at the University of Melbourne.