Marc Melendez Schofield

arXiv2026

Determining the Free-Carrier Fraction in 2D Perovskites using Power Dependent Photoluminescence

Determining the nature of the optical excited state (excitons or free carriers) in nanostructured materials is crucial for device design, as optoelectronic and photovoltaic technologies require different considerations regarding the optimized excited state dynamics. Power-dependent photoluminescence is widely used to distinguish between excitons and free carriers, but the classical power-law analysis oversimplifies the underlying physics when the exponent lies between the linear (pure excitons) and quadratic (pure free carriers) limits. In this work, we present a complete study enabling a direct and quantitative analysis of the free-carrier fraction based on power-dependent peak photoluminescence and placing its analysis in the context of the Saha-equation. We study Ruddlesden-Popper perovskites with varying thickness as a model system, as they cover a wide range of exciton binding energies and the full range of free carrier fractions. Our results agree with previously reported values for the exciton binding energies in these materials, confirming the reliability of this approach and providing a simple and effective tool for probing the nature of optically excited states in semiconductors with intermediate exciton binding energies. We demonstrate that our method allows probing spatial variations in the fraction of free charges near grain boundaries or edges at micrometer spatial resolution. Finally, our results highlight the importance of performing optical characterization under excitation densities relevant to realistic operating conditions, as higher fluences can artificially enhance exciton formation and distort excited-state interpretation under solar-fluence conditions.

cond-mat.mtrl-sci↗

arXiv2020

Hydrodynamics of quartz crystal microbalance experiments with liposome-DNA complexes

The quartz crystal microbalance (QCM) is widely used to study surface adsorbed molecules, often of biological significance. However, the relation between raw acoustic response (frequency shift $Δf$ and dissipation factor $ΔD$) and mechanical properties of the macromolecules still needs to be deciphered, particularly in the case of suspended discrete particles. We study the QCM response of suspended liposomes tethered to the resonator wall by double stranded DNA, with the other end attached to surface-adsorbed neutravidin through a biotin linker. Liposome radius and dsDNA contour length are comparable to the wave penetration depth ($δ\sim 100\ \mathrm{nm}$). Simulations, based on the immersed boundary method and an elastic network model for the liposome-DNA complex, are in good agreement with experimental results for POPC liposomes. We find that the added stress at the resonator surface, i.e. the impedance Z sensed by QCM, is dominated by the flow-induced liposome surface-stress, which propagates towards the resonator by viscous forces. QCM signals are extremely sensitive to the liposome's height distribution P(y) which depends on the actual number and mechanical properties of the tethers, in addition to the usual local attractive/repulsive chemical forces. Our approach helps in deciphering the role of hydrodynamics in acoustic sensing and revealing the role of parameters hitherto largely unexplored. A practical consequence would be the design of improved biosensors and detection schemes.

cond-mat.soft↗