Charlotte Clegg

Nanomechanical Mapping of Hydrated Rat Tail Tendon Collagen I Fibrils

Collagen fibrils play an important role in the human body, providing tensile strength to connective tissues. These fibrils are characterized by a banding pattern with a D-period of 67 nm. The proposed origin of the D-period is the internal staggering of tropocollagen molecules within the fibril, leading to gap and overlap regions and a corresponding periodic density fluctuation. Using an atomic force microscope high-resolution modulus maps of collagen fibril segments, up to 80 μm in length, were acquired at indentation speeds around 10(5) nm/s. The maps revealed a periodic modulation corresponding to the D-period as well as previously undocumented micrometer scale fluctuations. Further analysis revealed a 4/5, gap/overlap, ratio in the measured modulus providing further support for the quarter-staggered model of collagen fibril axial structure. The modulus values obtained at indentation speeds around 10(5) nm/s are significantly larger than those previously reported. Probing the effect of indentation speed over four decades reveals two distinct logarithmic regimes of the measured modulus and point to the existence of a characteristic molecular relaxation time around 0.1 ms. Furthermore, collagen fibrils exposed to temperatures between 50 and 62°C and cooled back to room temperature show a sharp decrease in modulus and a sharp increase in fibril diameter. This is also associated with a disappearance of the D-period and the appearance of twisted subfibrils with a pitch in the micrometer range. Based on all these data and a similar behavior observed for cross-linked polymer networks below the glass transition temperature, we propose that collagen I fibrils may be in a glassy state while hydrated.

Systematic study on the impact of water on the performance and stability of perovskite solar cells

The effects of water on the photovoltaic performance of methylammonium lead iodide perovskite devices has recently become a topic of interest within the perovskite community. However, existing studies report contrasting and sometimes contradictory observations. In this work, small concentrations of water, consistent with those present in air-exposed DMF, were systematically incorporated directly into the PbI2 precursor solution in order to investigate its effect on the performance of sequentially spin-coated, inverted perovskite solar cells. Increasing concentrations of water in the PbI2 solution were found to have a negative impact on photovoltaic performance. The addition of water was observed to exaggerate the scan-rate and directional-dependent hysteresis and introduce new transient behaviours in comparison to the “dry” devices. Interestingly, the addition of water was also found to improve the long-term device stability in comparison with devices fabricated from “dry” precursors.

Simultaneous observation of free and defect-bound excitons in CH 3 NH 3 PbI 3 using four-wave mixing spectroscopy

Solar cells incorporating organic-inorganic perovskite, which may be fabricated using low-cost solution-based processing, have witnessed a dramatic rise in efficiencies yet their fundamental photophysical properties are not well understood. The exciton binding energy, central to the charge collection process, has been the subject of considerable controversy due to subtleties in extracting it from conventional linear spectroscopy techniques due to strong broadening tied to disorder. Here we report the simultaneous observation of free and defect-bound excitons in CH3NH3PbI3 films using four-wave mixing (FWM) spectroscopy. Due to the high sensitivity of FWM to excitons, tied to their longer coherence decay times than unbound electron- hole pairs, we show that the exciton resonance energies can be directly observed from the nonlinear optical spectra. Our results indicate low-temperature binding energies of 13 meV (29 meV) for the free (defect-bound) exciton, with the 16 meV localization energy for excitons attributed to binding to point defects. Our findings shed light on the wide range of binding energies (2–55 meV) reported in recent years.

Four-Wave Mixing in Perovskite Photovoltaic Materials Reveals Long Dephasing Times and Weaker Many-Body Interactions than GaAs

Perovksite semiconductors have shown promise for low-cost solar cells, lasers and photodetectors, yet their fundamental photophysical properties are not well understood. Recent observations of a low exciton binding energy and evidence of hot phonon effects in the room temperature phase suggest that perovskites are much closer to inorganic semiconductors than the absorber layers in traditional organic photovoltaics, signaling the need for experiments that shed light on the placement of perovskite materials within the spectrum of semiconductors used in optoelectronics and photovoltaics. Here we use four-wave mixing (FWM) to contrast the coherent optical response of CH3NH3PbI3 thin films and crystalline GaAs. At carrier densities relevant for solar cell operation, our results show that carriers interact surprisingly weakly via the Coulomb interaction in perovskite, much weaker than in inorganic semiconductors. These weak many-body effects lead to a dephasing time in CH3NH3PbI3 ∼ 3× longer than in GaAs. Our r...

Carrier diffusion in thin-film CH3NH3PbI3 perovskite measured using four-wave mixing

We report the application of femtosecond four-wave mixing (FWM) to the study of carrier transport in solution-processed CH3NH3PbI3. The diffusion coefficient was extracted through direct detection of the lateral diffusion of carriers utilizing the transient grating technique, coupled with simultaneous measurement of decay kinetics exploiting the versatility of the boxcar excitation beam geometry. The observation of exponential decay of the transient grating versus interpulse delay indicates diffusive transport with negligible trapping within the first nanosecond following excitation. The in-plane transport geometry in our experiments enabled the diffusion length to be compared directly with the grain size, indicating that carriers move across multiple grain boundaries prior to recombination. Our experiments illustrate the broad utility of FWM spectroscopy for rapid characterization of macroscopic film transport properties.

Influence of Rashba splitting on carrier dynamics in organic-inorganic perovskites (Conference Presentation)

The lead halide hybrid perovskites have gained considerable attention in recent years due to their stellar performance as absorber layers in solution-processed solar cells, with efficiencies recently reaching over 22 percent [1]. Owing to their large spin-orbit coupling, these materials are also of interest for spintronic applications, in which the presence of lead may be less of an impediment to their adoption [2]. Measurements of spin dynamics in bulk CH3NH3PbI3-xClx have been reported in recent years [3,4,5], the spin-dependent optical Stark effect was demonstrated in 4F-PEPI [6], and a large Rashba effect has been predicted in both bulk and 2D perovskites [2], highlighting the need for further studies of the spin-related properties of these materials. Here we report spin-dependent measurements of carrier kinetics in butylammonium methylammonium lead iodide 2D perovskite and measurements of the coherent carrier response in 3D CH3NH3PbI3. Both experiments provide direct evidence of the impact of Rashba on the carrier kinetics in these systems, further supporting the potential for developing spin-optoelectronic devices using these materials. [1] https://www.nrel.gov/pv/assets/images/efficiency_chart.jpg. [2] M. Kepenekian and J. Even, J. Phys. Chem. Lett. 8, 3362 (2017). [3] D. Giovanni et al. Nano Lett. 15, 1553 (2015). [4] C. Zhang et al. Nat. Phys. 11, 427 (2015). [5] P. Odenthal et al. Nat. Phys. 13, 894 (2017). [6] D. Giovanni et al. Science Advances 2, e1600477 (2016).

Four-wave mixing response of solution-processed CH3NH3PbI3 thin films

The interest in perovskite-based solar cell absorber materials has skyrocketed in recent years due to the rapid rise in solar cell efficiency and the potential for cost reductions tied to solution-processed device fabrication. Due to complications associated with the presence of strong static and dynamic disorder in these organic-inorganic materials, the fundamental photophysical behavior of photo-excited charge carriers remains unclear. We apply four-wave mixing spectroscopy to study the charge carrier dynamics in CH3NH3PbI3 thin films. Our experiments reveal two discrete optical transitions below the band gap of the semiconductor with binding energies of 13 meV and 29 meV, attributed to free and defect-bound excitons respectively.