How do lattice vibrations mediate charge transfer across an interface? Answering this question requires experimental techniques that can visualize ultrafast lattice dynamics accompanying a charge-transfer event. I will discuss our recent work on type-II van der Waals heterostructures, which serve as excellent model systems for studying phonon-assisted electron transfer. Using femtosecond diffraction, we uncover a new mechanism for rapid energy sharing between monolayer semiconductors, involving charge transfer through a hybridized electronic state mediated by phonon emission in both layers[1]. Our work illuminates a novel route to control energy transport across atomic junctions. [1] Sood, Haber et al., Nat. Nanotechnol. (2023) https://doi.org/10.1038/s41565-022-01253-7

The transport of energy and information in semiconductors is limited by scattering between electronic carriers and lattice phonons, resulting in diffusive and lossy transport that curtails all semiconductor technologies. Using Re6Se8Cl2, a van der Waals (vdW) superatomic semiconductor, we demonstrate the formation of acoustic exciton-polarons, an electronic quasiparticle shielded from phonon scattering. We directly image polaron transport in Re6Se8Cl2 at room temperature and reveal quasi-ballistic, wavelike propagation sustained for nanoseconds and several microns. Shielded polaron transport leads to electronic energy propagation orders of magnitude greater than in other vdW semiconductors, exceeding even silicon over nanoseconds. We propose that, counterintuitively, quasi-flat electronic bands and strong exciton–acoustic phonon coupling are together responsible for the remarkable transport properties of Re6Se8Cl2, establishing a new path to ballistic room-temperature semiconductors.

Organic color centers (OCCs) are quantum defects synthetically installed on the surface of carbon nanotube semiconductors. The defects act as exciton traps where electrons and holes recombine radiatively to produce single photons in the shortwave infrared, even at room temperature. Unlike native defects, OCCs are molecularly tunable and can be synthetically controlled, potentially with atomic precision, using organic chemistry and physical means. These molecularly tunable defect color centers have opened exciting opportunities for chemistry, physics, materials science, biomedical engineering, and quantum technologies. In this talk, we will discuss our recent progress in this emerging field and provide an outlook on the rapidly expanding research and applications of these synthetic defects. In particular, the emitted photons carry local chemical information at the defect sites, opening new possibilities for pushing the limits of sensing and imaging. Time permitting, we will give examples to illustrate the potential applications of these quantum defects as biochemical sensors.

Our efforts to disentangle the link between molecular recognition and biochemical function are centered on the study of binding and dynamics of protein-RNA complexes at electrified interfaces. We are interested in the effects of electrostatics and local dynamics on the formation and stability of RNA-protein complexes. The use of complementary multimodal observations enables us to distinguish between distinct stages in the binding event between an RNA-binding protein and its target -- both of which are affected by interfacial electric fields. To follow dynamic events over multiple time scales, we combine mid-infrared surface plasmonics, time-resolved ultrafast fluorescence, and in situ electrochemical experiments at the surface of a degenerately doped wide-gap metal oxide.

Perovskite semiconductors are emerging low-cost materials for photovoltaics, light emitting devices and detectors. Because of the inclusion of high atomic numbered elements, perovskites are promising candidates for high efficiency X-ray sensing. In this talk, I will discuss the properties of perovskite semiconductors for X-ray and visible photon sensing. Firstly, we report a long carrier diffusion length in 2D perovskite single crystals, assisted by the shallow trap and de-trapping process. Next, we show that such a long diffusion length ensures a full charge collection after charge ionization, which is beneficial for detectors for X-ray and other photons. In addition, we have found the shallow trap also extend the carrier transport lifetime that facilitate a charge multiplication in the detector driven under high voltages. Such a process introduces a photo conductivity gain, leading to an unusually high X-ray and visible photon sensing efficiency. A high gain can be also achieved by building a hetero-structured device. Interfacing perovskites with a high mobility graphene channel can also multiplicate the photo-generated carriers. With a hetero-structured device, we show a high X-ray sensitivity over 108 µCGy-1cm-2.

In this work, we probe the relationship between perovskite material microstructure and the nature of the photogenerated polarons. We prepare formamidinium lead iodide quantum dot films in two forms with distinctive inter-QD interactions. Using a combination of time-resolved spectroscopies, we observe a competition between carrier thermalization into polaronic excitons and ultrafast hot-carrier transport between QDs, with the balance between channels set by the excess photon energy and connectivity between QDs. These results underscore the importance of microscopic structure in steering the ultrafast processes which determines the fate of photoexcitations in perovskite materials.

Ruddlesden-Popper (2D) perovskites offer exciting possibilities for various applications that leverage their electronic and thermodynamic properties, including solar cells, light-emitting diodes, transistors, thermal energy storage, and barocaloric cooling. While numerous methods exist for controlling their electronic properties, only a few techniques exist for fine-tuning their thermodynamic properties. In this study, we present a novel approach to controlling the phase transition temperature of RP 2D perovskites through the incorporation of alkyl organic cations with varying chain lengths. Using temperature-dependent grazing incidence wide-angle X-ray scattering (GIWAXS) and photoluminescence (PL) spectroscopy, we also demonstrate how the phase transition in the organic layer affects the structure of the inorganic lattice, influencing PL intensity and wavelength. Our findings offer valuable design principles for controlling phase transitions in 2D perovskites, which can enable the development of solid-solid phase change materials and barocaloric cooling applications.

Coherent nonlinear spectroscopy offers us a window into the system-bath interactions in materials. Specifically, the spectral lineshapes can reveal the nature and dynamics of the environmental fluctuations surrounding the system of interest. Here we will discuss how stochastic non-equilibrium exciton dynamics manifest in the peculiar lineshapes and how they provide mechanistic insights into the nature of exciton-phonon and exciton-exciton interactions in nanostructured derivatives of metal halide perovskites. Despite the success of such classical optical probes in unveiling the many-body physics in materials, we will elaborate on the ambiguities still present in the resultant photophysical models that stem primarily due to the high excitation intensities used in the measurements. We will also discuss alternative experimental methodologies based on quantum entangled photons, which may offer superior signal to noise ratio and thus enabling the measurement of many-body interactions at close to single photon excitation densities.

The spin of ground and excited states in organic semiconductors gives the playground for photophysics in optoelectronics: with applications from solar cells to light-emitting diodes (LEDs) using energy and charge transfer processes. Spin controls the functional behaviour: where the longstanding issue in organic LEDs is 1:3 ratio of singlet and triplet excitons from charge recombination. Here magnetic resonance studies can probe the key spin conversion mechanisms between luminescent and dark states. I will present studies of luminescent organic radicals and the doublet-spin energy manifold, which can be interfaced with singlet and triplet excitons to improve performance in near-infrared OLEDs.

Achieving ballistic energy flow in materials at room temperature is a long-standing goal that could unlock lossless energy harvesting and wave-based information technologies. I will describe two avenues to achieve ballistic transport by harnessing strong interactions between coherent and incoherent excitations in 2D materials. The first is to leverage strong interactions between photons and semiconductor excitons, yielding part-light part-matter particles known as polaritons. The second is to leverage strong interactions between electrons and delocalized phonons, forming coherent polarons. In both cases, we image the propagation of these particles using unique ultrafast microscopies on femtosecond and few-nanometer scales.

In organic and organic/inorganic hybrid materials, the role of the exciton reservoir (i.e., uncoupled excitons) towards populating exciton-polariton states —emerging when an excitonic transition strongly couples with a microcavity optical mode— is not well understood. Here, we identify many-body processes in the exciton reservoir by probing the time-resolved nonlinear photoluminescence of polaritons in an organic dye (1,6,7,12-bay-substituted perylene-diimide derivative) and a Ruddlesden-Popper (PEA)2PbI4 perovskite. We observe that, in the dye, exciton-exciton annihilation is hindered in the strong light-matter coupling regime and, in (PEA)2PbI4, Auger recombination obstructs the population of exciton-polaritons via radiative pumping by the emission of the exciton reservoir.

Exciton-polaritons created from coupling electronic states of the molecule with quantized radiation field inside a Fabry-Pérot (FP) optical cavity can lead to altered cavity-mediated chemical reactions and provide a platform for studying quantum electrodynamics in chemical physics. We discuss the photophysical properties of polaritons formed with two-dimensional cadmium selenide nanoplatelets inside an optical cavity operating in the strong coupling regime at room temperature with upper and lower polariton Rabi splitting energy of 80 meV.

Singlet fission, in which two triplet excitons are formed from one photon, has the potential to increase the power conversion efficiency of solar cells. While singlet fission has been studied extensively over the past decade, the role of vibrational coupling remain unclear [1]. Femtosecond stimulated Raman spectroscopy (FSRS) is an ultrafast nonlinear optical technique that investigates vibrational structures of molecules providing both high temporal resolution (∼100 fs) and reasonable spectral resolution (10cm−1). In this three−pulse technique, excited and ground state vibrational spectra are obtained to gain structural information about singlet and triplet kinetics in a fast singlet fission process. We have investigated the excited-state dynamics of 6,13-bis triiso-propylsilylethynyl (TIPS) Pentacene in concentrated solution to determine the role of excimer and aggregate formation in singlet fission solutions. Using transient absorption, we find that the mechanism of singlet fission remains dominated by diffusive encounters in highly-concentration solutions (Dvořák et al. [2]). Our research extends to track vibrations during the singlet fission process using FSRS. Here we present the time-resolved vibrational spectrum of highly-concentrated TIPS-Pentacene solutions. The molecular structural evolution is probed by monitoring vibrational changes in Raman transitions after a variable time delay. The data is compared with DFT computational methods to identify the excited state vibrational species. These results will be used to gain further insights into the singlet fission process. Kim, W., & Musser, A. J. (2021). Tracking ultrafast reactions in organic materials through vibrational coherence: vibronic coupling mechanisms in singlet fission. Advances in Physics: X, 6(1), 1918022. Dvořák, M., Prasad, S. K., Dover, C. B., Forest, C. R., Kaleem, A., MacQueen, R. W., ... & Schmidt, T. W. (2021). Singlet fission in concentrated TIPS-pentacene solutions: The role of excimers and aggregates. Journal of the American Chemical Society, 143(34), 13749-13758.

SF is a multiexciton generation process, in which two triplet excitons form from a single absorbed photon. Since geometry limit of the covalent dimers, more than half of triplet state will be lost in a dephasing process. We demonstrate that individual triplet excitons can be extracted directly from a bound triplet pair (yield >50%) prior to dephasing. The harvesting process is not dependent on the net multiplicity of the triplet pair state, suggesting that an explicit dissociation step is not a requirement for using triplet pairs to do chemical or electrical work.

When molecular vibrational modes strongly couple to virtual states of photonic modes, new molecular vibrational polariton states are formed, along with a large population of dark reservoir modes. The polaritons are much like the bonding and antibonding molecular orbitals when atomic orbitals form molecular bonds, while the dark modes are like non-bonding orbitals. Because the polariton states are half-matter and half-light, whose energy is shifted from the parental states, polaritons are predicted to modify chemistry under thermally-activated conditions, led to an exciting and emerging field referred as polariton chemistry that could potentially shift paradigms in chemistry. Despite several published results supporting this concept, the chemical physics and mechanism of polariton chemistry remain elusive. One reason for this challenge is that previous works cannot differentiate polaritons from dark modes. This limitation makes delineating the contributions from polaritons and dark states to chemistry difficult. However, this level of insight is critical for developing a solid mechanism for polariton chemistry to design and predict the outcome of strong coupling with any given reaction. My group addressed the challenge of differentiating the dynamics of polaritons and dark modes by ultrafast two-dimensional infrared (2D IR) spectroscopy. Specifically, (1) we found that polaritons can facilitate intra- and intermolecular vibrational energy transfer, opening a pathway to control vibrational energy flow in liquid phase molecular systems; (2) By studying a single-step isomerization event, we verified that indeed polaritons could modify chemical dynamics under strong coupling conditions, but in contrast, the dark modes behave like uncoupled molecules and do not change the dynamics. This finding confirmed the central concept of polariton chemistry – polaritons modified the potential energy landscape of reactions. The result also clarified the role of dark modes, which lays a critical foundation for designing cavities for future polariton chemistry.

Modifications of chemical reaction rates via strong light-matter coupling at mid-infrared regime has attracted significant interest in physics and chemistry in recent years. However, despite numerous efforts, there is currently no general theory that can fully describe the available experimental evidence. We implement an open quantum system model to describe the suppression of the intracavity reaction rate for alcoholysis of phenyl isocyanate with cyclohexanol in infrared Fabry-Perot cavities. The model considers the three molecular modes observed in the infrared absorption spectrum of phenyl isocyanate in the region of interest. The results point out that suppression of intracavity reaction rates is modified when a cavity is resonant with specific molecular modes of the reactant [1]. We derive analytical expressions using a reduced model that explains the role of light-matter coherences and molecular disorder in the modifications of intracavity chemical processes. Our findings significantly improve our understanding of cavity-modified chemistry by tuning cavity modes with different molecular modes. [1] W.Ahn, J.F. Triana, F. Recabal, F. Herrera, B.S. Simpkins . Chemrxiv, wb6vs (2022)

Light-Matter interaction is essential in chemistry, physics, and material science. Modifying potential energy surfaces due to light-matter interactions leads to new chemical reaction pathways. A deep understanding of the fundamental details of light-matter interactions usually requires simulations of non-adiabatic transitions among a dense manifold of excited states. We recently developed several formalisms to address the light-matter interaction-induced non-adiabatic phenomena. First, I will discuss our recent developments in theory and numerical methods for simulating plasmon-mediated chemical reactions and the physical insights obtained. Then, we will present our recent developments of theoretical methods for calculating polariton states and simulating polariton dynamics, including QED coupled-cluster theory and ab-initio multiple cloning methods.

Supramolecular self-assembly is showcased as a powerful tool for controlling exciton interactions and light polarization for organic optoelectronics. We will discuss a supramolecular pseudo-cube assembled from PDI chromophores with enhanced brightness, in which we observe an excited multimer state using ultrafast spectroscopy. Moreover, we show the precise engineering of DPP chromophore stacks, of which an exceptionally bright Pd2L2L’2 (>50% PLQY) benefits from an intra-assembly FRET process. Finally, the concept of chirality for light polarization control using supramolecular engineering will be explored.

Organic and perovskites photovoltaic devices have many advantageous properties including tailorable light absorption, low embodied energy manufacturing, structural conformality, and low material toxicity. Apart from solar cells, these properties also make these materials attractive for applications such as indoor light-harvesting cells and photodetectors. A critical parameter limiting the performance of these diode devices is the dark saturation current which limits the open-circuit voltage of organic solar cells and the detectivity of organic photodetectors. It is known that the dark saturation current is strongly limited by non-radiative processes resulting in dark saturation currents orders of magnitude higher than expected for radiative band-to-band transitions; however, the origin of these non-radiative processes is still debated. Here, we show that the dark saturation current in organics is fundamentally mediated by mid-gap trap states.[1,2] This midgap traps are also present in perovskites and impose additional voltage loss which can reduced upon passivation.[3] This new insight is generated by a universal trend observed for a large set of organic bulk heterojunction systems and substantiated by sensitive external quantum efficiency and temperature-dependent current measurements.[4] These findings have important implications for organics and perovskites, providing new insight into the origin of non-radiative losses in light-harvesting applications such as organic and perovskite solar cells and photodiodes. References: 1. Sandberg, Oskar J., Armin, Ardalan, et al. "Mid-gap trap state-mediated dark current in organic photodiodes." Nature Photonics (2023): 1-7. 2. Zarrabi, Nasim; Armin, Ardalan et al. "Charge-generating mid-gap trap states define the thermodynamic limit of organic photovoltaic devices." Nature Communications 11.1 (2020): 5567. 3. Warby, Jonathan; Stolterfoht, Martin; Armin, Ardalan et al. "Understanding performance limiting interfacial recombination in pin perovskite solar cells." Advanced Energy Materials 12.12 (2022): 2103567. 4. Zarrabi, Nasim; Armin. Ardalan et al. "Subgap Absorption in Organic Semiconductors." The Journal of Physical Chemistry Letters 14 (2023): 3174-3185.

Layered double perovskites (LDPs) have emerged as a promising alternative to traditional, single-metal perovskites due to their improved stability, reduced toxicity, and unexplored chemical space. Among the several potential applications of LDPs, their promising photophysical properties make them attractive for various optoelectronic applications. This talk will discuss our recent research on a family of highly emissive LDPs and our efforts to understand their properties and emission mechanism. Our studies include the exploration of the crystal structure, optical properties, and photoluminescence mechanism of these materials. We will also discuss the potential applications of these highly emissive LDPs as light-emitting materials for displays and solid-state lighting. Our findings pave the way for developing new and efficient LDP-based optoelectronic devices.

The recently developed combination of vacuum electrospray deposition (ESD) and scanning tunnelling microscopy (STM) is here compared to commonly-used analytical techniques GPC and NMR. We study three DPP-based polymers, of which only two produce clearly-resolved NMR spectra. Our subnanometre-resolved images provide, in one single experiment, detailed information about complete mass distributions and exact polymer sequences, including the identification of polymerisation defects. We, therefore, show that ESD-STM represents a new powerful analytical tool, successfully benchmarked against NMR and GPC when these methods are viable, and that can be used to characterise with equal accuracy polymers that are inaccessible by traditional techniques.

In this research we present a novel method to measure local optical dichroism in opaque crystal powder suspensions using the photoacoustic effect. Our method is based upon the laser speckle contrast technique, a novel technique to perform photoacoustic measurements that do not require contact with the sample. The main novelty of our work is the development of a simple statistical approach for measuring the chirality of crystal suspensions using the photoacoustic effect, which does not require arranging the crystals with a specific orientation on surfaces. A model chiral system was used to demonstrate our method, we have used Cobalt doped L-Histidine crystals that are photoacoustic active and established our ability to measure their optical dichroism in solution under completely random orientation.

Aggregation in conjugated polymers is a well-recognized driver of performance in organic opto-electronic devices. In particular, good device performance is correlated with processing methods that also minimize non-radiative traps that quench emission in thin films. Here we compare the efficacy of various solvent systems in producing weakly versus highly emissive aggregates in the well-studied polymer poly(3-hexylthiophene) or P3HT and show that many systems that appear highly quenched in bulk solution are strongly emissive in the solid state. Microscopy, transient dynamical measurements, and structural studies to probe the electronic and structural differences between weakly and highly emissive aggregates are described.

Upconversion (UC) of NIR photons into visible photons can improve photovoltaics technologies and enable passive night vision. However, current solid-state triplet-triplet annihilation UC devices suffer from low absorption, low energy transfer rates, and highly parasitic back transfer processes which lead to low external quantum efficiencies (EQE). We propose the introduction of a “blocker layer” which can mitigate FRET-based back transfer to improve the EQE. We demonstrate the use of 5-tetracene carboxylic acid (TCA) as a ligand/blocker layer to improve EQE by 3-5x. Finally, we deconvolve the mechanism of improvement through spectroscopic comparison of the traditional and our novel UC devices.

Singlet fission, in which two triplet excitons are formed from one photon, has the potential to increase the power conversion efficiency of solar cells. While singlet fission has been studied extensively over the past decade, the role of vibrational coupling remain unclear [1]. Femtosecond stimulated Raman spectroscopy (FSRS) is an ultrafast nonlinear optical technique that investigates vibrational structures of molecules providing both high temporal resolution (∼100 fs) and reasonable spectral resolution (10cm−1). In this three−pulse technique, excited and ground state vibrational spectra are obtained to gain structural information about singlet and triplet kinetics in a fast singlet fission process. We have investigated the excited-state dynamics of 6,13-bis triiso-propylsilylethynyl (TIPS) Pentacene in concentrated solution to determine the role of excimer and aggregate formation in singlet fission solutions. Using transient absorption, we find that the mechanism of singlet fission remains dominated by diffusive encounters in highly-concentration solutions (Dvořák et al. [2]). Our research extends to track vibrations during the singlet fission process using FSRS. Here we present the time-resolved vibrational spectrum of highly-concentrated TIPS-Pentacene solutions. The molecular structural evolution is probed by monitoring vibrational changes in Raman transitions after a variable time delay. The data is compared with DFT computational methods to identify the excited state vibrational species. These results will be used to gain further insights into the singlet fission process. Kim, W., & Musser, A. J. (2021). Tracking ultrafast reactions in organic materials through vibrational coherence: vibronic coupling mechanisms in singlet fission. Advances in Physics: X, 6(1), 1918022. Dvořák, M., Prasad, S. K., Dover, C. B., Forest, C. R., Kaleem, A., MacQueen, R. W., ... & Schmidt, T. W. (2021). Singlet fission in concentrated TIPS-pentacene solutions: The role of excimers and aggregates. Journal of the American Chemical Society, 143(34), 13749-13758.

Organic radical molecules are promising for spintronic and quantum information devices due to microsecond spin relaxation lifetimes and strong optical interactions for excitonic states. In this talk, I will set out work combining radical doublet molecules with organic and hybrid-semiconductor triplet systems where spin polarisation has been demonstrated, towards the creation of a novel optical-spin interface. A scheme for transferring spin polarisation from triplet and sensitiser systems to the radical by energy transfer will be outlined. Energy transfer from triplet to doublet excitons will be demonstrated and the mechanisms elucidated by time- and temperature-dependent optical and magnetic resonance spectroscopy.