arXiv subjects

K. Nogajewski

Publications and source records attributed to K. Nogajewski.

At least 19 recordsLinked to original sources

arXiv2023

Raman scattering excitation in monolayers of semiconducting transition metal dichalcogenides

Raman scattering excitation (RSE) is an experimental technique in which the spectrum is made up by sweeping the excitation energy when the detection energy is fixed. We study the low-temperature ($T$=5~K) RSE spectra measured on four high quality monolayers (ML) of semiconducting transition metal dichalcogenides (S-TMDs), $i.e.$ MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$, encapsulated in hexagonal BN. The outgoing resonant conditions of Raman scattering reveal an extraordinary intensity enhancement of the phonon modes, which results in extremely rich RSE spectra. The obtained spectra are composed not only of Raman-active peaks, $i.e.$ in-plane E$'$ and out-of-plane A$'_1$, but the appearance of 1$^{st}$, 2$^{nd}$, and higher-order phonon modes is recognised. The intensity profiles of the A$'_1$ modes in the investigated MLs resemble the emissions due to neutral excitons measured in the corresponding PL spectra for the outgoing type of resonant Raman scattering conditions. Furthermore, for the WSe$_2$ ML, the A$'_1$ mode was observed when the incoming light was in resonance with the neutral exciton line. The strength of the exciton-phonon coupling (EPC) in S-TMD MLs strongly depends on the type of their ground excitonic state, $i.e.$ bright or dark, resulting in different shapes of the RSE spectra. Our results demonstrate that RSE spectroscopy is a powerful technique for studying EPC in S-TMD MLs.

cond-mat.mes-hall↗

arXiv2020

Excitonic complexes in $n$-doped WS$_2$ monolayer

We investigate the origin of emission lines apparent in the low-temperature photoluminescence spectra of $n$-doped WS$_2$ monolayer embedded in hexagonal BN layers using external magnetic fields and first-principles calculations. Apart from the neutral A exciton line, all observed emission lines are related to the negatively charged excitons. Consequently, we identify emissions due to both the bright (singlet and triplet) and dark (spin- and momentum-forbidden) negative trions as well as the phonon replicas of the latter optically-inactive complexes. The semi-dark trions and negative biexcitons are distinguished. Based on their experimentally extracted and theoretically calculated $g$-factors, we identify three distinct families of emissions due to exciton complexes in WS$_2$: bright, intravalley and intervalley dark. The $g$-factors of the spin-split subbands in both the conduction and valence bands are also determined.

cond-mat.mes-hall↗

arXiv2020

Resonance and antiresonance in Raman scattering in GaSe and InSe crystals

The temperature effect on the Raman scattering efficiency is investigated in $\varepsilon$-GaSe and $\gamma$-InSe crystals. We found that varying the temperature over a broad range from 5 K to 350 K permits to achieve both the resonant conditions and the antiresonance behaviour in Raman scattering of the studied materials. The resonant conditions of Raman scattering are observed at about 270 K under the 1.96 eV excitation for GaSe due to the energy proximity of the optical band gap. In the case of InSe, the resonant Raman spectra are apparent at about 50 K and 270 K under correspondingly the 2.41 eV and 2.54 eV excitations as a result of the energy proximity of the \mbox{so-called} B transition. Interestingly, the observed resonances for both materials are followed by an antiresonance behaviour noticeable at higher temperatures than the detected resonances. The significant variations of phonon-modes intensities can be explained in terms of electron-phonon coupling and quantum interference of contributions from different points of the Brillouin zone

cond-mat.mtrl-sci↗

arXiv2020

Neutral and charged dark excitons in monolayer WS$_2$

Low temperature and polarization resolved magneto-photoluminescence experiments are used to investigate the properties of dark excitons and dark trions in a monolayer of WS$_2$ encapsulated in hexagonal BN (hBN). We find that this system is an $n$-type doped semiconductor and that dark trions dominate the emission spectrum. In line with previous studies on WSe$_2$, we identify the Coulomb exchange interaction coupled neutral dark and grey excitons through their polarization properties, while an analogous effect is not observed for dark trions. Applying the magnetic field in both perpendicular and parallel configurations with respect to the monolayer plane, we determine the g-factor of dark trions to be $g\sim$-8.6. Their decay rate is close to 0.5 ns, more than 2 orders of magnitude longer than that of bright excitons.

cond-mat.mes-hall↗

arXiv2020

Valley polarization of singlet and triplet trions in WS$_2$ monolayer in magnetic fields

The spectral signatures associated with different negatively charged exciton complexes (trions) in a WS$_2$ monolayer encapsulated in hBN, are analyzed from low temperature and polarization resolved reflectance contrast (RC) and photoluminescence (PL) experiments, with an applied magnetic field. Based on results obtained from the RC experiment, we show that the valley Zeeman effect affects the optical response of both the singlet and the triplet trion species through the evolution of their energy and of their relative intensity, when applying an external magnetic field. Our analysis allows us to estimate a free electron concentration of $\sim 1.3 \cdot 10^{11}$ cm$^{-2}$. The observed evolutions based on PL experiments on the same sample are different and can hardly be understood within the same simple frame highlighting the complexity of relaxation processes involved in the PL response.

cond-mat.mes-hall↗

arXiv2019

The effect of metallic substrates on the optical properties of monolayer MoSe$_{2}$

Atomically thin materials, like semiconducting transition metal dichalcogenides (S-TMDs), are highly sensitive to the environment. This opens up an opportunity to externally control their properties by changing their surroundings. We investigate the effect of several metallic substrates on the optical properties of MoSe$_2$ monolayer (ML) deposited on top of them with photoluminescence and reflectance contrast techniques. The optical spectra of MoSe$_{2}$ MLs deposited on Pt, Au, Mo and Zr have distinctive metal-related lineshapes. In particular, a substantial variation in the intensity ratio and the energy separation between a negative trion and a neutral exciton is observed. It is shown that using metals as substrates affects the doping of S-TMD MLs. The explanation of the effect involves the Schottky barrier formation at the interface between the MoSe$_{2}$ ML and the metallic substrates. The alignment of energy levels at the metal/semiconductor junction allows for the transfer of charge carriers between them. We argue that a proper selection of metallic substrates can be a way to inject appropriate types of carriers into the respective bands of S-TMDs.

cond-mat.mes-hall↗

arXiv2019

Breathing modes in few-layer MoTe$_2$ activated by h-BN encapsulation

The encapsulation of few-layer transition metal dichalcogenides (TMDs) in hexagonal boron nitride (h-BN) is known to improve significantly their optical and electronic properties. However, it may be expected that the h-BN encapsulation may affect also vibration properties of TMDs due to an atomically flat surface of h-BN layers. In order to study its effect on interlayer interactions in few-layer TMDs, we investigate low-energy Raman scattering spectra of bi- and trilayer MoTe$_2$. Surprisingly, three breathing modes are observed in the Raman spectra of the structures deposited on or encapsulated in h-BN as compared to a single breathing mode for the flakes deposited on a SiO$_2$/Si substrate. The shear mode is not affected by changing the MoTe$_2$ environment. The emerged structure of breathing modes is ascribed to the apparent interaction between the MoTe$_2$ layer and the bottom h-BN flake. The structure becomes visible due to a high-quality surface of the former flake. Consequently, the observed triple structure of breathing modes originates from the combination modes due to interlayer and layer-substrate interactions. Our results confirm that the h-BN encapsulation affects substantially vibration properties of layered materials.

cond-mat.mes-hall↗

arXiv2019

Exciton-polaritons in multilayer WSe$_2$ in a planar microcavity

Due to high binding energy and oscillator strength, excitons in thin flakes of transition metal dichalcogenides constitute a perfect foundation for realizing a strongly coupled light-matter system. In this paper we investigate mono- and few-layer WSe$_2$ flakes encapsulated in hexagonal boron nitride and incorporated into a planar dielectric cavity. We use an open cavity design which provides tunability of the cavity mode energy by as much as 150 meV. We observe a strong coupling regime between the cavity photons and the neutral excitons in direct-bandgap monolayer WSe$_2$, as well as in few-layer WSe$_2$ flakes exhibiting indirect bandgap. We discuss the dependence of the exciton's oscillator strength and resonance linewidth on the number of layers and predict the exciton-photon coupling strength.

cond-mat.mes-hall↗

arXiv2019

Energy spectrum of two-dimensional excitons in a non-uniform dielectric medium

We demonstrate that, in monolayers (MLs) of semiconducting transition metal dichalcogenides, the $s$-type Rydberg series of excitonic states follows a simple energy ladder: $\epsilon_n=-Ry^*/(n+\delta)^2$, $n$=1,2,\ldots, in which $Ry^*$ is very close to the Rydberg energy scaled by the dielectric constant of the medium surrounding the ML and by the reduced effective electron-hole mass, whereas the ML polarizability is only accounted for by $\delta$. This is justified by the analysis of experimental data on excitonic resonances, as extracted from magneto-optical measurements of a high-quality WSe$_2$ ML encapsulated in hexagonal boron nitride (hBN), and well reproduced with an analytically solvable Schr\"odinger equation when approximating the electron-hole potential in the form of a modified Kratzer potential. Applying our convention to other, MoSe$_2$, WS$_2$, MoS$_2$ MLs encapsulated in hBN, we estimate an apparent magnitude of $\delta$ for each of the studied structures. Intriguingly, $\delta$ is found to be close to zero for WSe$_2$ as well as for MoS$_2$ monolayers, what implies that the energy ladder of excitonic states in these two-dimensional structures resembles that of Rydberg states of a three-dimensional hydrogen atom.

cond-mat.mes-hall↗

arXiv2019

Probing and manipulating valley coherence of dark excitons in monolayer WSe$_2$

Monolayers of semiconducting transition metal dichalcogenides are two-dimensional direct-gap systems which host tightly-bound excitons with an internal degree of freedom corresponding to the valley of the constituting carriers. Strong spin-orbit interaction and the resulting ordering of the spin-split subbands in the valence and conduction bands makes the lowest-lying excitons in WX$_2$ (X~being S or Se) spin-forbidden and optically dark. With polarization-resolved photoluminescence experiments performed on a WSe$_2$ monolayer encapsulated in a hexagonal boron nitride, we show how the intrinsic exchange interaction in combination with the applied in-plane and/or out-of-plane magnetic fields enables one to probe and manipulate the valley degree of freedom of the dark excitons.

cond-mat.mes-hall↗

arXiv2018

Dimensional crossover of acoustic phonon lifetime in $2H$-MoSe$_2$

A time-resolved observation of coherent interlayer longitudinal acoustic phonons in 2$H$-MoSe$_2$ is reported. A femtosecond pump-probe technique is used to investigate the evolution of the energy loss of these vibrational modes in a wide selection of MoSe$_2$ flakes with different thicknesses ranging from bilayer up to the bulk limit. By directly analysing the temporal decay of the modes, we can clearly distinguish an abrupt crossover related to the acoustic mean free path of the phonons in a layered system, and the constraints imposed to the acoustic decay channels when reducing the dimensionality. Loses intrinsic to the low dimensionality of single or few layer materials impose critical limitations for their use in optomechanical and optoelectronic devices.

cond-mat.mes-hall↗

arXiv2018

Fine structure of $\mathrm{K}$-excitons in multilayers of transition metal dichalcogenides

Reflectance and magneto-reflectance experiments together with theoretical modelling based on the $\mathbf{k\cdot p}$ approach have been employed to study the evolution of direct bandgap excitons in MoS$_2$ layers with a thickness ranging from mono- to trilayer. The extra excitonic resonances observed in MoS$_2$ multilayers emerge as a result of the hybridization of Bloch states of each sub-layer due to the interlayer coupling. The properties of such excitons in bi- and trilayers are classified by the symmetry of corresponding crystals. The inter- and intralayer character of the reported excitonic resonances is fingerprinted with the magneto-optical measurements: the excitonic $g$-factors of opposite sign and of different amplitude are revealed for these two types of resonances. The parameters describing the strength of the spin-orbit interaction are estimated for bi- and trilayer MoS$_2$.

cond-mat.mes-hall↗

arXiv2018

Valley polarization of exciton-polaritons in monolayer WSe2 in a tunable microcavity

Monolayer transition metal dichalcogenides, known for exhibiting strong excitonic resonances, constitute a very interesting and versatile platform for investigation of light-matter interactions. In this work we report on a strong coupling regime between excitons in monolayer WSe2 and photons confined in an open, voltage-tunable dielectric microcavity. The tunability of our system allows us to extend the exciton-polariton state over a wide energy range and, in particular, to bring the excitonic component of the lower polariton mode into resonance with other excitonic transitions in monolayer WSe2. With selective excitation of spin-polarized exciton-polaritons we demonstrate the valley polarization when the polaritons from the lower branch come into resonance with a bright trion state in monolayer WSe2 and valley depolarization when they are in resonance with a dark trion state.

cond-mat.mes-hall↗

arXiv2018

Raman scattering from the bulk inactive out-of-plane B$^{1}_{2\text{g}}$ mode in few-layer MoTe$_{2}$

Raman scattering from the out-of-plane vibrational modes (A$_{1\text{g}}$/A'$_{1}$), which originate from the bulk-inactive out-of-plane B$^{1}_{2\text{g}}$ mode, are studied in few-layer MoTe$_{2}$. Temperature-dependent measurements reveal a doublet structure of the corresponding peaks in the Raman scattering spectra of tetralayer and pentalayer samples. A strong enhancement of their lower energy components is recorded at low temperature for 1.91 eV and 1.96 eV laser excitation. We discuss the attribution of the peaks to the inner modes of the respective Raman-active vibrations. The temperature evolution of their intensity strongly suggests a resonant character of the employed excitation, which leads to the mode enhancement at low temperature. The resonance of the laser light with the singularity of the electronic density of states at the $M$ point of the Brillouin zone in MoTe$_{2}$ is proposed to be responsible for the observed effects.

cond-mat.mtrl-sci↗

arXiv2018

Orbital, spin and valley contributions to Zeeman splitting of excitonic resonances in MoSe$_2$, WSe$_2$ and WS$_2$ monolayers

We present a comprehensive optical study of the excitonic Zeeman effects in transition metal dichalcogenide monolayers, which are discussed comparatively for selected materials: MoSe$_2$, WSe$_2$ and WS$_2$. We introduce a simple semi-phenomenological description of the magnetic field evolution of individual electronic states in fundamental sub-bands by considering three additive components: valley, spin and orbital terms. We corroborate the validity of the proposed description by inspecting the Zeeman-like splitting of neutral and charged excitonic resonances in absorption-type spectra. The values of all three terms are estimated based on the experimental data, demonstrating the significance of the valley term for a consistent description of magnetic field evolution of optical resonances, particularly those corresponding to charged states. The established model is further exploited for discussion of magneto-luminescence data. We propose an interpretation of the observed large g-factor values of low energy emission lines, due to so-called bound/localized excitons in tungsten based compounds, based on the brightening mechanisms of dark excitonic states.

cond-mat.mes-hall↗

arXiv2018

Singlet and triplet trions in WS$_2$ monolayer encapsulated in hexagonal boron nitride

Embedding a WS$_2$ monolayer in flakes of hexagonal boron nitride allowed us to resolve and study the photoluminescence response due to both singlet and triplet states of negatively charged excitons (trions) in this atomically thin semiconductor. The energy separation between the singlet and triplet states has been found to be relatively small reflecting rather weak effects of the electron-electron exchange interaction for the trion triplet in a WS$_2$ monolayer, which involves two electrons with the same spin but from different valleys. Polarization-resolved experiments demonstrate that the helicity of the excitation light is better preserved in the emission spectrum of the triplet trion than in that of the singlet trion. Finally, the singlet (intravalley) trions are found to be observable even at ambient conditions whereas the emission due to the triplet (intervalley) trions is only efficient at low temperatures.

cond-mat.mes-hall↗

arXiv2017

Impact of environment on dynamics of exciton complexes in a WS$_2$ monolayer

Scientific curiosity to uncover original optical properties and functionalities of atomically thin semiconductors, stemming from unusual Coulomb interactions in the two-dimensional geometry and multi-valley band structure, drives the research on monolayers of transition metal dichalcogenides (TMDs). While recent works ascertained the exotic energetic schemes of exciton complexes in TMDs, we here employ four-wave mixing microscopy to indicate that their subpicosecond dynamics is determined by the surrounding disorder. Focusing on a monolayer WS$_2$, we observe that exciton coherence is lost primarily due to interaction with phonons and relaxation processes towards optically dark excitonic states. Notably, when temperature is low and disorder weak excitons large coherence volume results in huge oscillator strength, allowing to reach the regime of radiatively limited dephasing and we observe long valley coherence. We thus elucidate the crucial role of exciton environment in the TMDs on its dynamics and show that revealed mechanisms are ubiquitous within that family.

cond-mat.mes-hall↗

arXiv2017

Sub-bandgap voltage electroluminescence and magneto-oscillations in a WSe2 light-emitting van der Waals heterostructure

We report on experimental investigations of an electrically driven WSe2 based light-emitting van der Waals heterostructure. We observe a threshold voltage for electroluminescence significantly lower than the corresponding single particle band gap of monolayer WSe2. This observation can be interpreted by considering the Coulomb interaction and a tunneling process involving excitons, well beyond the picture of independent charge carriers. An applied magnetic field reveals pronounced magneto-oscillations in the electroluminescence of the free exciton emission intensity with a 1/B-periodicity. This effect is ascribed to a modulation of the tunneling probability resulting from the Landau quantization in the graphene electrodes. A sharp feature in the differential conductance indicates that the Fermi level is pinned and allows for an estimation of the acceptor binding energy.

cond-mat.mes-hall↗