arXiv subjects

Kameron R. Hansen

Publications and source records attributed to Kameron R. Hansen.

4 recordsLinked to original sources

arXiv2026

Structural tuning of reduced exciton mass in layered HOIP compounds: Causation vs. correlation

Reduced exciton mass ($\mu$) was recently reported to correlate strongly with a framework distortion in a series of nine single-layer (2D) metal-halide perovskite (HOIP) compounds. Specifically, $\mu$ was observed to increase in tandem with an alternating PbI4 octahedral tilt about an in-plane axis. In this work, we use group representation theory to decompose the observed framework distortions into displacive symmetry modes of a common high-symmetry parent framework. We find that all nine distorted frameworks involve linear combinations of the same six symmetry modes, which have been reported to contribute to the framework distortions of a wide range of HOIP compounds. We show that these modes have highly correlated impacts on the band structure. To differentiate causation from correlation, we vary the amplitude of each mode independently and use density-functional theory to determine the resulting electronic band structures, from which $\mu$ is extracted. We find that bond-transverse displacements of the equatorial halide atoms increase $\mu$, while bond-transverse displacements of the apical halide atoms decrease it. Bond-axis displacements appear to have little or no effect on $\mu$. Our results demonstrate three new structure-property relationships, revealing a promising new avenue for exciton engineering in layered perovskite materials.

cond-mat.mtrl-sci↗

arXiv2023

Exciton properties: learning from a decade of measurements on halide perovskites and transition metal dichalcogenides

The exciton binding energy ($E_b$) is a key parameter that governs the physics of many optoelectronic devices. At their best, trustworthy and precise measurements of $E_b$ challenge theoreticians to refine models, are a driving force in advancing the understanding a material system, and lead to efficient device design. At their worst, inaccurate $E_b$ measurements lead theoreticians astray, sew confusion within the research community, and hinder device improvements by leading to poor designs. This review article seeks to highlight the pros and cons of different measurement techniques used to determine $E_b$, namely, temperature-dependent photoluminescence, resolving Rydberg states, electroabsorption, magnetoabsorption, scanning tunneling spectroscopy, and fitting the optical absorption. Due to numerous conflicting $E_b$ values reported for halide perovskites (HP) and transition metal dichalcogenides (TMDC) monolayers, an emphasis is placed on highlighting these measurements in attempt to reconcile the variance between different measurement techniques. By considering the published data en masse, we argue the experiments with the clearest indicators are in agreement on the following values: ~350 - 450 meV for TMDC monolayers between SiO$_2$ and vacuum, ~150 - 200 meV for hBN-encapsulated TMDC monolayers, ~200 - 300 meV for common lead-iodide 2D HPs, and ~10 meV for methylammonium lead iodide.

cond-mat.mtrl-sci↗

arXiv2021

Franz-Keldysh and Stark Effects in Two-Dimensional Metal Halide Perovskites

As the field of metal halide perovskites (MHP) matures, state-of-the-art techniques to measure basic properties such as the band gap and exciton binding energy continue to produce inconsistent values. This issue is persistent even for 2D MHPs wherein the large separation between exciton and continuum states should make such measurements more straightforward. In this study, we revert to the established theory of a 2D Wannier exciton in a uniform electric field to analyze the electroabsorption response of an archetypal 2D MHP system, phenethylammonium lead iodide (PEA2PbI4). The high level of agreement between the electroabsorption simulation and measurement allows for a deepened understanding of the exciton's redshift according to the quadratic Stark effect and the continuum wavefunction leaking according to the Franz-Keldysh effect. We find the field-dependency of each of these effects to be rich with information, yielding measurements of the exciton's Bohr radius, transition dipole moment, polarizability, and reduced effective mass. Most importantly, the exciton binding energy is unambiguously determined with 2% uncertainty. The high precision of these new measurement methods opens the opportunity for future studies to accurately determine the influence of chemical and environmental factors on the optoelectronic properties of MHPs and thereby increase the tunability of this important class of materials.

cond-mat.mes-hall↗

arXiv2019

Direct Determination of Band Gap Renormalization in Photo-Excited Monolayer MoS2

A key feature of monolayer semiconductors, such as transition-metal dichalcogenides, is the poorly screened Coulomb potential, which leads to large exciton binding energy (Eb) and strong renormalization of the quasiparticle bandgap (Eg) by carriers. The latter has been difficult to determine due to cancellation in changes of Eb and Eg, resulting in little change in optical transition energy at different carrier densities. Here we quantify bandgap renormalization in macroscopic single crystal MoS2 monolayers on SiO2 using time and angle resolved photoemission spectroscopy (TR-ARPES). At excitation density above the Mott threshold, Eg decreases by as much as 360 meV. We compare the carrier density dependent Eg with previous theoretical calculations and show the necessity of knowing both doping and excitation densities in quantifying the bandgap.

cond-mat.str-el↗