arXiv subjects

Julia Kucharek

Publications and source records attributed to Julia Kucharek.

6 recordsLinked to original sources

arXiv2025

Magnetic field induced polarization enhancement in the photoluminescence of MBE-grown WSe$_2$ layers

We report an experimental study of the magnetic-field dependence of the optically pumped valley polarization in an epitaxial tungsten diselenide (WSe$_2$) monolayer grown by molecular-beam epitaxy (MBE) on a hexagonal boron nitride (hBN) substrate. Circularly polarized photoluminescence (PL) measurements reveal that applying a weak out-of-plane magnetic field, on the order of 0.1 T, dramatically increases the effectiveness of the optical orientation of the emission associated with defect-bound localized excitons. We compare the obtained results with the earlier studies on the reference exfoliated monolayers, discussing both qualitative similarity as well as quantitative differences. Our observations are further supplemented by the results of time-resolved PL measurements, which confirm the pseudospin relaxation time of approximately 25 ps, a value significantly shorter than the $\approx$100 ps previously reported for mechanically exfoliated samples.

cond-mat.mtrl-sci↗

arXiv2025

Photoluminescence efficiency of MBE-grown MoSe$_2$ monolayers featuring sharp excitonic lines and diverse grain structures

Recent studies have demonstrated that using h-BN as a substrate for the growth of transition metal dichalcogenides can significantly reduce excitonic linewidths. However, many other optical parameters still require optimization. In this work, we present a detailed study of the low-temperature photoluminescence efficiency of MBE-grown MoSe$_2$ monolayers on h-BN substrates, comparing them to state-of-the-art exfoliated monolayers encapsulated in h-BN. We demonstrate that a quantitative comparison between samples requires accounting for interference effects and Purcell enhancement or suppression of the emission. By accounting for these effects in both photoluminescence and Raman signals, we show that the overall intrinsic luminescence efficiency is proportional to the sample coverage. Consequently, we find that exciton diffusion and edge effects are negligible in spectroscopy of MBE-grown samples, even for nanometer-sized crystals.

cond-mat.mes-hall↗

arXiv2025

$WSe_2$ Monolayers Grown by Molecular Beam Epitaxy on hBN

A three-step process was developed for growing high-quality, optically uniform WSe2 monolayers by molecular beam epitaxy (MBE) with advantage of using hexagonal boron nitride (hBN). The process was optimized to maximize the efficiency of photoluminescence and promote formation of hexagonal WSe2 domains. Atomic force microscopy (AFM) was employed to estimate the dispersion of WSe2 hexagonal domains orientation. Monolayer character of the film was identified using optical methods and verified with high-resolution transmission electron microscopy (TEM) cross-section. Temperature-and-magnetic-field-dependent studies revealed the behaviour of exciton complexes to be analogical to that of exfoliated counterparts. Direct growth on hBN combined with uniform optical response proves this WSe2 superior to mechanically exfoliated WSe2 in terms of convenience of use and reproducibility. Provided results establish a significant progress in optical quality of epitaxially grown transition metal dichalcogenides (TMDs) monolayers and fabrication of large-scale functional devices.

cond-mat.mtrl-sci↗

arXiv2024

Twisted MoSe2 Homobilayer Behaving as a Heterobilayer

Heterostructures (HSs) formed by the transition-metal dichalcogenides (TMDCs) materials have shown great promise in next-generation optoelectronic and photonic applications. An artificially twisted HS, allows us to manipulate the optical, and electronic properties. With this work, we introduce the understanding of the complex energy transfer (ET) process governed by the dipolar interaction in a twisted molybdenum diselenide (MoSe2) homobilayer without any charge-blocking interlayer. We fabricated an unconventional homobilayer (i.e., HS) with a large twist angle by combining the chemical vapor deposition (CVD) and mechanical exfoliation (Exf.) techniques to fully exploit the lattice parameters mismatch and indirect/direct (CVD/Exf.) bandgap nature. This effectively weaken the charge transfer (CT) process and allows the ET process to take over the carrier recombination channels. We utilize a series of optical and electron spectroscopy techniques complementing by the density functional theory calculations, to describe a massive photoluminescence enhancement from the HS area due to an efficient ET process. Our results show that the electronically decoupled MoSe2 homobilayer is coupled by the ET process, mimicking a 'true' heterobilayer nature.

cond-mat.mes-hall↗

arXiv2023

Molecular beam epitaxy growth of cadmium telluride structures on hexagonal boron nitride

We investigate the feasibility of epitaxial growth of a three-dimensional semiconductor on a two-dimensional substrate. In particular, we report for the first time on molecular beam epitaxy growth of cadmium telluride (CdTe) quantum wells on hexagonal boron nitride (hBN). The presence of the quantum wells is confirmed by photoluminescence measurements conducted at helium temperatures. Growth of quantum wells on two dimensional, almost perfectly flat hBN appears to be very different from growth on bulk substrates, in particular it requires 70-100$^\circ$C lower temperatures.

cond-mat.mtrl-sci↗

arXiv2023

Molecular Beam Epitaxy Growth of Transition Metal Dichalcogenide (Mo,Mn)Se$_2$ on 2D, 3D and polycrystalline substrates

Magnetic doping of 2D materials such as Transition Metal Dichalcogenides is promising for the enhancement of magneto-optical properties, as it was previously observed for 3D diluted magnetic semiconductors. To maximize the effect of magnetic ions, they should be incorporated into the crystal lattice of 2D material rather than form separated precipitates. This work shows a study on incorporating magnetic manganese ions into the MoSe$_2$ monolayers using molecular beam epitaxy. We test growth on various substrates with very different properties: polycrystalline SiO$_2$ on Si, exfoliated 2D hexagonal Boron Nitride flakes (placed on SiO$_2$ / Si), monocrystalline sapphire, and exfoliated graphite (on tantalum foil). Although atomic force microscopy images indicate the presence of MnSe precipitates, but at the same time, various techniques reveal effects related to alloying MoSe$_2$ with Mn: Raman scattering and photoluminescence measurements show energy shift related to the presence of Mn, scanning transmission microscopy shows Mn induced partial transformation of 1H to 1T^\prime phase. Above effects evidence partial incorporation of Mn into the MoSe$_2$ layer.

cond-mat.mtrl-sci↗