arXiv subjects

Marc Meléndez

Publications and source records attributed to Marc Meléndez.

10 recordsLinked to original sources

arXiv2026

The Hidden Cost of Alloying: Disorder-Driven Transport Collapse in TMDs

Alloying in two dimensional semiconductors is widely used to tune bandgaps, yet its implications for charge and energy transport remain poorly understood. Here, we investigate MoS2xSe2(1-x) alloys as a model system to study the interplay between composition, thickness, and disorder. Optical transitions and valence band dispersions evolve continuously with both stoichiometry and number of layers, with negligible bandgap bowing and a composition dependent attenuation of thickness driven renormalization. In contrast, time resolved spatial mapping of photoexcited carriers reveals a pronounced and asymmetric collapse of carrier diffusivity at intermediate compositions, which cannot be accounted for by changes in effective mass or band alignment, and instead emerges from strong real space fluctuations in the local energetic landscape generated by random chalcogen substitution. Microscopic simulations reproduce the experimental trends and show that the character of disorder depends critically on the direction of alloying, producing either scattering barriers or deep trapping sites. Together, these results demonstrate that transport in TMD alloys is governed by disorder physics, overlooked by conventional optical and photoemission probes at equilibrium. Our findings establish transport as a stringent metric of electronic quality and high-light intrinsic limitations in the usage of TMD alloys for layered semiconductor devices.

cond-mat.mtrl-sci↗

arXiv2026

Exciton Transport in Disordered Perovskite Nanocrystal Solids

Solution-processed thin films of colloidal lead halide perovskite (LHP) nanocrystals (NCs) show great potential for the implementation into optoelectronic devices such as light-emitting diodes (LEDs), lasers, and solar cells. However, these hybrid LHP NC solids exhibit non-negligible size and shape polydispersity, which introduces both structural and energetic disorder. Here, we resolve the exciton dynamics in space, time, and energy to elucidate the impact of different forms of disorder (structural and energetic) on exciton transport. We show that the disorder depends sensitively on the length of the alkylamine ligand used in the synthesis. While shorter alkyl chain lengths lead to high polydispersity, longer alkyl chains lead to more monodispersed and smaller particles where quantum confinement becomes more pronounced and, consequently, lead to increased energetic disorder. Strikingly, we find that exciton transport is less efficient in NC solids with long alkyl chain ligands, despite having a significantly more monodisperse ensemble. This demonstrates that energetic disorder, rather than structural disorder, is the dominant factor for predicting exciton transport within these materials. These findings reveal the critical role of ligand engineering in designing high-performance optoelectronic devices based on hybrid LHP NCs, providing new insights into energy transport dynamics in disordered systems and highlighting the versatility of these materials for advanced photonic and optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2024

Responding to objections made by Sorin Coşofreţ concerning kinetic molecular theory: an illustration of how to teach physics students to evaluate pseudoscientific work

Sorin Coşofreţ has argued that kinetic molecular theory does not correctly describe thermal equilibrium in ideal gases and has provided some examples that purportedly show that the standard laws imply consequences that contradict experimental results. This paper considers a few of the examples in detail and concludes that Coşofreţ's analysis is erroneous. Furthermore, although it does not disprove any of the other claims made by Coşofreţ in criticising thermodynamics and many other fields of physics, it does call his whole project into question, as Coşofreţ exhibits a lack of relevant training, limited knowledge of key facts and previous research, and a disposition to favour his own intuitions above theoretical and experimental checks. The exercise below illustrates some of the problems commonly encountered in pseudoscientific approaches, and provides a few tips on how to teach aspiring scientists about how to evaluate such works for themselves.

physics.ed-ph↗

arXiv2023

Boosting the efficiency of transient photoluminescence microscopy using cylindrical lenses

Transient Photoluminescence Microscopy (TPLM) allows for the direct visualization of carrier transport in semiconductor materials with sub nanosecond and few nanometer resolution. The technique is based on measuring changes in the spatial distribution of a diffraction limited population of carriers using spatiotemporal detection of the radiative decay of the carriers. The spatial resolution of TPLM is therefore primarily determined by the signal-to-noise-ratio (SNR). Here we present a method using cylindrical lenses to boost the signal acquisition in TPLM experiments. The resulting asymmetric magnification of the photoluminescence emission of the diffraction limited spot can increase the collection efficiency by more than a factor of 10, significantly reducing acquisition times and further boosting spatial resolution.

physics.app-ph↗

arXiv2021

Halide mixing inhibits exciton transport in two-dimensional perovskites despite phase purity

Metal-halide perovskites are a versatile material platform for light-harvesting and light-emitting applications as their variable chemical composition allows the optoelectronic properties to be tailored to specific applications. Halide mixing is one of the most powerful techniques to tune the optical bandgap of metal-halide perovskites across wide spectral ranges. However, halide mixing has commonly been observed to result in phase segregation, which reduces excited-state transport and limits device performance. While the current emphasis lies on the development of strategies to prevent phase segregation, it remains unclear how halide mixing may affect excited-state transport even if phase purity is maintained. In this work, we study excitonic excited-state transport in phase pure mixed-halide 2D perovskites. Using transient photoluminescence microscopy, we show that, despite phase purity, halide mixing inhibits exciton transport in these materials. We find a significant reduction even for relatively low alloying concentrations, with bromide-rich perovskites being particularly sensitive to the introduction of iodide ions. Performing Brownian dynamics simulations, we are able to reproduce our experimental results and attribute the decrease in diffusivity to the energetically disordered potential landscape that arises due to the intrinsic random distribution of alloying sites. Our results suggest that even in the absence of phase segregation, halide mixing may still impact carrier transport due to the local intrinsic inhomogeneities in the energy landscape.

physics.app-ph↗

arXiv2020

Mapping the Trap-State Landscape in 2D Metal-Halide Perovskites using Transient Photoluminescence Microscopy

Transient microscopy is of vital importance in understanding the dynamics of optical excited states in optoelectronic materials, as it allows for a direct visualization of the movement of energy carriers in space and time. Important information on the influence of trap-states can be obtained using this technique, typically observed as a slow-down of the energy transport as carriers are trapped at defect sites. To date, however, studies of the trap-state dynamics have been mostly limited to phenomenological descriptions of the early time-dynamics. In this report, we show how long-acquisition-time transient photoluminescence microscopy can be used to provide a detailed map of the trap-state landscape in 2D perovskites, in particular when used in combination with transient spectroscopy. We reveal anomalous spatial dynamics of excitons in 2D perovskites, which cannot be explained with existing models for trap limited exciton transport that only account for a single trap type. Instead, using a continuous diffusion model and performing Brownian dynamics simulations, we show that this behavior can be explained by accounting for a distinct distribution of traps in this material. Our results highlight the value of transient microscopy as a complementary tool to more common transient spectroscopy techniques in the characterization of the excited state dynamics in semiconductors.

cond-mat.mtrl-sci↗

arXiv2020

Exciton diffusion in two-dimensional metal-halide perovskites

Two-dimensional perovskites, in which inorganic layers are stabilized by organic spacer molecules, are attracting increasing attention as a more robust analogue to the conventional three-dimensional metal-halide perovskites. However, reducing the perovskite dimensionality alters their optoelectronic properties dramatically, yielding excited states that are dominated by bound electron-hole pairs known as excitons, rather than by free charge carriers common to their bulk counterparts. Despite the growing interest in two-dimensional perovskites for both light harvesting and light emitting applications, the full impact of the excitonic nature on their optoelectronic properties remains unclear, particularly regarding the spatial dynamics of the excitons within the two-dimensional (2D) plane. Here, we present direct measurements of in-plane exciton transport in single-crystalline layered perovskites. Using time-resolved fluorescence microscopy, we show that excitons undergo an initial fast, intrinsic normal diffusion through the crystalline plane, followed by a transition to a slower subdiffusive regime as excitons get trapped. Interestingly, the early intrinsic exciton diffusivity depends sensitively on the exact composition of the perovskite, such as the choice of organic spacer. We attribute these changes in exciton transport properties to strong exciton-phonon interactions and the formation of large exciton-polarons. Our findings provide a clear design strategy to optimize exciton transport in these systems.

physics.app-ph↗

arXiv2019

Optofluidic control of the dispersion of nanoscale dumbbells

Previous research has shown that gold nanoparticles immersed in water in an optical vortex lattice formed by the perpendicular intersection of two standing light waves with a π/2 rad phase difference will experience enhanced dispersion that scales with the intensity of the incident laser. We show that flexible nanoscale dumbbells (created by attaching two such gold particles by means of a polymer chain) in the same field display different types of motion depending on the chain length and field intensity. We have not disregarded the secondary optical forces due to light scattering. The dumbbells may disperse, rotate or remain trapped. For some values of the parameters, the (enhanced) dispersion possesses a displacement distribution with exponential tails, making the motion anomalous, though Brownian.

physics.optics↗

arXiv2014

Approximating the entire spectrum of nonequilibrium steady state distributions using relative entropy: An application to thermal conduction

We show that distribution functions of nonequilibrium steady states (NESS) evolving under a slowly varying protocol can be accurately obtained from limited data and the closest known detailed state of the system. In this manner, one needs to perform only a few detailed experiments to obtain the nonequilibrium distribution function for the entire gamut of nonlinearity. We achieve this by maximizing the relative entropy functional (MaxRent), which is proportional to the Kullback-Leibler distance from a known density function, subject to constraints supplied by the problem definition and new measurements. MaxRent is thus superior to the principle of maximum entropy (MaxEnt), which maximizes Shannon's informational entropy for estimating distributions but lacks the ability of incorporating additional prior information. The MaxRent principle is illustrated using a toy model of $ϕ^4$ thermal conduction consisting of a single lattice point. An external protocol controlled position-dependent temperature field drives the system out of equilibrium. Two different thermostatting schemes are employed: the Hoover-Holian deterministic thermostat (which produces multifractal dynamics under strong nonlinearity) and the Langevin stochastic thermostat (which produces phase space-filling dynamics). Out of the 80 possible states produced by the protocol, we assume that 4 states are known to us in detail, one of which is used as input into MaxRent at a time. We find that MaxRent accurately approximates the phase space density functions at all values of the protocol even when the known distribution is far away. MaxEnt, however, is unable to capture the fine details of the phase space distribution functions. We expect this method to be useful in other external protocol driven nonequilibrium cases as well.

cond-mat.stat-mech↗

arXiv2013

Comment on "Logarithmic Oscillators: Ideal Hamiltonian Thermostats" [arXiv 1203.5968]

Campisi, Zhan, Talkner and Hänggi have recently proposed a novel Hamiltonian thermostat which they claim may be used both in simulations and experiments [arXiv:1203.5968v4]. We show, however, that this is not possible due to the length and time scales involved, which depend exponentially on the total energy of the system. The implementation suggested by Campisi et alii implies equilibration times greater than the age of the universe for systems with more than a few dozen particles.

cond-mat.stat-mech↗