Dark trions govern the temperature-dependent optical absorption and emission of doped atomically thin semiconductors

We perform absorption and photoluminescence spectroscopy of trions in hBN-encapsulated WSe2, WS2, MoSe2, and MoS2 monolayers, depending on temperature. The different trends for W- and Mo-based materials are excellently reproduced considering a Fermi-Dirac distribution of bright and dark trions. We find a dark trion, XD- 19 meV below the lowest bright trion, X1- in WSe2 and WS2. In MoSe2, X-D lies 6 meV above X1-, while XD- and X1- almost coincide in MoS2. Our results agree with GW-BSE ab-initio calculations and quantitatively explain the optical response of doped monolayers with temperature.

Ashish Arora, Nils Kolja Wessling, Thorsten Deilmann, Till Reichenauer, Paul Steeger, Piotr Kossacki, Marek Potemski, Steffen Michaelis de Vasconcellos, Michael Rohlfing, Rudolf Bratschitsch

Flipping exciton angular momentum with chiral phonons in MoSe2/WSe2 heterobilayers

Identifying quantum numbers to label elementary excitations is essential for the correct description of light-matter interaction in solids. In monolayer semiconducting transition metal dichalcogenides (TMDs) such as MoSe2 or WSe2, most optoelectronic phenomena are described well by labelling electron and hole states with the spin projection along the normal to the layer (Sz). In contrast, for WSe2/MoSe2 interfaces recent experiments show that taking Sz as quantum number is not a good approximation, and spin mixing needs to be always considered. Here we argue that the correct quantum number for these systems is not Sz, but the z-component of the total angular momentum -- Jz = Lz + Sz -- associated to the C3 rotational lattice symmetry, which assumes half-integer values corresponding modulo 3 to distinct states. We validate this conclusion experimentally through the observation of strong intervalley scattering mediated by chiral optical phonons that -- despite carrying angular momentum 1 -- cause resonant intervalley transitions of excitons, with an angular momentum difference of 2.

A. Delhomme, D. Vaclavkova, A. Slobodeniuk, M. Orlita, M. Potemski, D.M. Basko, K. Watanabe, T. Taniguchi, D. Mauro, C. Barreteau, E. Giannini, A.F. Morpurgo, N. Ubrig, C. Faugeras

Narrow excitonic lines and large scale homogeneity of transition metal dichalcogenide grown by MBE on hBN

Monolayer transition-metal dichalcogenides (TMDs) manifest exceptional optical properties related to narrow excitonic resonances. However, these properties have been so far explored only for structures produced by techniques inducing considerable large-scale inhomogeneity. In contrast, techniques which are essentially free from this disadvantage, such as molecular beam epitaxy (MBE), have to date yielded only structures characterized by considerable spectral broadening, which hinders most of the interesting optical effects. Here, we report for the first time on the MBE-grown TMD exhibiting narrow and resolved spectral lines of neutral and charged exciton. Moreover, our material exhibits unprecedented high homogeneity of optical properties, with variation of the exciton energy as small as ±0.16 meV over a distance of tens of micrometers. Our recipe for MBE growth is presented for MoSe2 and includes the use of atomically flat hexagonal boron nitride substrate. This recipe opens a possibility of producing TMD heterostructures with optical quality, dimensions, and homogeneity required for optoelectronic applications.

W. Pacuski, M. Grzeszczyk, K. Nogajewski, A. Bogucki, K. Oreszczuk, J. Kucharek, K.E. Połczynska, B. Seredynski, R. Bozek, T. Taniguchi, K. Watanabe, J. Sadowski, T. Kazimierczuk, M. Potemski, and P. Kossacki
Nano Lett. 20, 3058 (2020)

Ultra-long-working-distance spectroscopy of single nanostructures with aspherical solid immersion microlenses

In light science and applications, equally important roles are played by efficient light emitters/detectors and by the optical elements responsible for light extraction and delivery. The latter should be simple, cost effective, broadband, versatile and compatible with other components of widely desired micro-optical systems. Ideally, they should also operate without high-numerical-aperture optics. Here, we demonstrate that all these requirements can be met with elliptical microlenses 3D printed on top of light emitters. Importantly, the microlenses we propose readily form the collected light into an ultra-low divergence beam (half-angle divergence below 1°) perfectly suited for ultra-long-working-distance optical measurements (600 mm with a 1-inch collection lens), which are not accessible to date with other spectroscopic techniques. Our microlenses can be fabricated on a wide variety of samples, including semiconductor quantum dots and fragile van der Waals heterostructures made of novel two-dimensional materials, such as monolayer and few-layer transition metal dichalcogenides.

Aleksander Bogucki, Łukasz Zinkiewicz, Magdalena Grzeszczyk, Wojciech Pacuski, Karol Nogajewski, Tomasz Kazimierczuk, Aleksander Rodek, Jan Suffczyński, Kenji Watanabe, Takashi Taniguchi, Piotr Wasylczyk, Marek Potemski, Piotr Kossacki
Light: Science & Applications 9, 48 (2020)

The effect of metallic substrates on the optical properties of monolayer MoSe2

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. Photoluminescence and reflectance contrast techniques are employed to investigate the effect of metallic substrates on optical properties of MoSe2 monolayer (ML). The optical spectra of MoSe2 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 MoSe2 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.

M. Grzeszczyk, M. R. Molas, K. Nogajewski, M. Bartoš, A. Bogucki, C. Faugeras, P. Kossacki, A. Babiński & M. Potemski
Sci Rep 10, 4981 (2020)

Temperature dependence of photoluminescence lifetime of atomically thin WSe2 layer

At cryogenic temperatures, the photoluminescence (PL) spectrum of monolayer WSe2 features a number of lines related to the recombination of so-called localized excitons (LEs). The intensity of these lines strongly decreases with increasing temperature. In order to understand the mechanism behind this phenomenon we carried out a time-resolved experiment, which revealed a similar trend in the PL decay time. Our results identify the opening of additional non-radiative relaxation channels as a primary cause of the observed temperature quenching of the LEs' PL.

A. Łopion, M. Goryca, T. Smoleński, K. Oreszczuk, K. Nogajewski, M. R. Molas, M. Potemski, P. Kossacki
Nanotechnology 31, 135002 (2020)

Breathing modes in few-layer MoTe2 activated by h-BN encapsulation

The encapsulation of few-layer transition metal dichalcogenide (TMD) structures in hexagonal boron nitride (h-BN) is known to improve their optical properties, which is of crucial importance for their applications. In order to study the effect of encapsulation on interlayer interactions in few-layer TMDs the low-energy Raman scattering spectrum of bi- and trilayer MoTe2 is investigated. Three breathing modes are observed in the spectra of these few-layer MoTe2 structures deposited on/encapsulated in h-BN as compared to one breathing mode for the flakes deposited on an SiO2/Si substrate. Conversely, the shear mode is not affected by changing the substrate. We relate the emerged structure of breathing modes to the interaction of MoTe2 with the h-BN substrate. The interaction slightly affects the energy of the main breathing mode and contributes to the combination modes due to interlayer and layer-substrate interactions. We also show that the h-BN substrate determines the Raman spectrum. The interaction between MoTe2 and the top h-BN layer has a negligible effect on the low-frequency Raman scattering spectrum.

M. Grzeszczyk, M. R. Molas, B. Bartoš, K. Nogajewski, M. Potemski, A. Babiński
Appl. Phys. Lett., accepted for publication

Exciton-polaritons in multilayer WSe2 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 WSe2 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 WSe2, as well as in few-layer WSe2 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.

M. Król, K. Rechcińska, K. Nogajewski, M. Grzeszczyk, K. Łempicka, R. Mirek, S. Piotrowska, K. Watanabe, T. Taniguchi, M. R. Molas, M. Potemski, J. Szczytko, B. Piętka
2D Materials 7, 015006 (2020).

Upconverted electroluminescence via Auger scattering of interlayer excitons in van der Waals heterostructures

The intriguing physics of carrier-carrier interactions, which likewise aff ect the operation of light emitting devices, stimulate the research on semiconductor structures at high densities of excited carriers, a limit reachable at large pumping rates or in systems with long-lived electron-hole pairs. By electrically injecting carriers into WSe2/MoS2 type-II heterostructures which are indirect in real and k-space, we establish a large population of typical optically silent interlayer excitons. Here, we reveal their emission spectra and show that the emission energy is tunable by an applied electric eld. When the population is further increased by suppressing the radiative recombination rate with the introduction of an hBN spacer between WSe2 and MoS2, Auger-type and exciton-exciton annihilation processes become important. These processes are traced by the observation of an upconverted emission demonstrating that excitons gaining energy in non-radiative Auger processes can be recovered and recombine radiatively.

J. Binder, J. Howarth, F. Withers, M.R. Molas, T. Taniguchi, K. Watanabe, C. Faugeras, A. Wysmolek, M. Danovich, V. Falko, A. Geim, K. Novoselov, M. Potemski, A. Kozikov
Nature Communications 10, 2335 (2019)

The lifetime of interlayer breathing modes of few-layer 2H-MoSe2 membranes

A time-resolved observation of coherent interlayer longitudinal acoustic phonons in thin layers of 2H-MoSe2 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 MoSe2 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 on the acoustic decay channels when reducing the dimensionality. For thicker samples, the main acoustic attenuation mechanism is attributed to the scattering of the acoustic modes with thermal phonons. For samples thinner than ∼20 molecular layers, the predominant damping mechanism is ascribed to the effects of surface asperity. Losses intrinsic to the low dimensionality of single or few layer materials impose critical limitations for their use in optomechanical and optoelectronic devices.

P. Soubelet, A. A. Reynoso, A. Fainstein, K. Nogajewski, M. Potemski, C. Faugeras, A. E. Bruchhausen
Nanoscale 11, 10446-10453 (2019)

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: ϵn=Ry/(n+δ)2, n=1,2,..., in which Ry is 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 δ. This is justified by the analysis of experimental data on excitonic resonances, as extracted from magneto-optical measurements of a high-quality WSe2 ML encapsulated in hexagonal boron nitride (hBN), and well reproduced with an analytically solvable Schrödinger equation when setting the electron-hole potential in the form of a modified Kratzer potential. Applying our convention to other, MoSe2, WS2, MoS2 MLs encapsulated in hBN, we estimate an apparent magnitude of δ for each of the studied structures. Intriguingly, δ is found to be close to zero for WSe2 as well as for MoS2 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.

M. R. Molas, A. O. Slobodeniuk, K. Nogajewski, M. Bartos, Ł. Bala, A. Babiński, K. Watanabe, T. Taniguchi, C. Faugeras, M. Potemski
Phys. Rev. Lett. 123, 136801 (2019)

Probing and manipulating valley coherence of dark excitons in monolayer WSe2

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 WX2 (X being S or Se) spin-forbidden and optically dark. This results in their long lifetime, making them potentially interesting for valleytronics. With polarization-resolved photoluminescence experiments performed on a WSe2 monolayer encapsulated in a hexagonal boron nitride, we show how the intrinsic exchange interaction in combination with the applied in-plane and perpendicular magnetic fields enables one to probe and manipulate the valley degree of freedom of the dark excitons.

M. R. Molas, A. O. Slobodeniuk, T. Kazimierczuk, K. Nogajewski, M. Bartos, P. Kapuściński, K. Oreszczuk, K. Watanabe, T. Taniguchi, C. Faugeras, P. Kossacki, D. M. Basko, M. Potemski
Phys. Rev. Lett. 123, 096803 (2019)

Time-resolved magneto-Raman study of carrier dynamics in low Landau levels of graphene

We study the relaxation dynamics of the electron system in graphene flakes under Landau quantization regime using a novel approach of time-resolved Raman scattering. The non-resonant character of the experiment allows us to analyze the field dependence of the relaxation rate. Our results clearly evidence sharp increase in the relaxation rate upon the resonance between the energy of the Landau transition and the G-band and shed new light on relaxation mechanism of the Landau-quantized electrons in graphene beyond the previously studied Auger scattering.

T. Kazimierczuk, A. Bogucki, T. Smoleński, M. Goryca, C. Faugeras, P. Machnikowski, M. Potemski, P. Kossacki
Phys. Rev. B 100, 075401 (2019)

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.

M. Król, K. Lekenta, R. Mirek, K. Łempicka, D. Stephan, K. Nogajewski, M. R. Molas, A. Babiński, M. Potemski, J. Szczytko, B. Piętka
Nanoscale, accepted for publication

Fine structure of K-excitons in multilayers of transition metal dichalcogenides

Reflectance and magneto-reflectance experiments together with theoretical modelling based on the kp approach have been employed to study the evolution of direct bandgap excitons in MoS2 layers with a thickness ranging from mono- to trilayer. The extra excitonic resonances observed in MoS2 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 MoS2.

A. O. Slobodeniuk, Ł. Bala, M. Koperski, M. R. Molas, P. Kossacki, K. Nogajewski, M. Bartos, K. Watanabe, T. Taniguchi, C. Faugeras, M. Potemski
2D Materials 6, 025026 (2019)

Tuning carrier concentration in a superacid treated MoS2 monolayer

The effect of the superacid treatment on optical properties of MoS2 monolayers is investigated. Photoluminescence, reflectance contrast and Raman scattering spectroscopy were employed in a broad range of temperature. It has been found that the treatment results in systematic quenching of the trion emission/absorption and the redshift of the neutral exciton emission/absorption associated with both A and B excitonic resonances in monolayer MoS2. The trion complex related to the B exciton in monolayer MoS2 has also been identified. The defect-related emission observed at low temperatures also disappears from the spectrum as a result of the treatment. Our observations are attributed to the effective passivation of defects on the sample surface. The passivation leads to the vanishing carrier density, which affects the out-of-plane electric field in the structure. The observed tuning of carrier density influences also strongly the Raman scattering in the MoS2 monolayer. The enhancement of the Raman scattering by the resonance with neutral exciton in the vicinity of the A resonance affects both out-of-plane A1 and in-plane E modes. On the contrary, while the excitation is in resonance with a corresponding trion, Raman scattering can be hardly distinguished. These results confirm a role of the excitonic charge state on the resonant effect of the excitation on the Raman scattering in transition metal dichalcogenides.

M. R. Molas, K. Gołasa, Ł. Bala, K. Nogajewski, M. Bartos, M. Potemski, A. Babiński
Scientific Reports 9, 1989 (2019)

Zeeman spectroscopy of excitons and hybridization of electronic states in few-layer WSe2, MoSe2 and MoTe2

Monolayers and multilayers of semiconducting transition metal dichalcogenides (TMDCs) offer an ideal platform to explore valley-selective physics with promising applications in valleytronics and information processing. Here we manipulate the energetic degeneracy of the and valleys in few-layer TMDCs. We perform high-field magneto-reflectance spectroscopy on WSe2, MoSe2, and MoTe2 crystals of thickness from monolayer to the bulk limit under magnetic fields up to applied perpendicular to the sample plane. Because of a strong spin-layer locking, the ground state A excitons exhibit a monolayer-like valley Zeeman splitting with a negative g-factor, whose magnitude increases monotonically when crystal thickness decreases from bulk to a monolayer. Using kp calculations, we demonstrate that the observed dependence of g-factors for different materials is well accounted for by hybridization of electronic states in the and valleys. The mixing of the valence and conduction band states induced by the interlayer interaction decreases the g-factor with increasing layer number. The effect is the largest for MoTe2, followed by MoSe2, and the smallest for WSe2.

A. Arora, M. Koperski, A. Slobodeniuk, K. Nogajewski, R. Schmidt, R. Schneider, M.R. Molas, S.M. de Vasconcellos, R. Bratschitsch, M. Potemski
2D Materials 6, 015010 (2019)

Orbital, spin and valley contributions to Zeeman splitting of excitonic resonances in MoSe2, WSe2 and WS2 monolayers

We present a comprehensive optical study of the excitonic Zeeman effects in transition metal dichalcogenide monolayers, which are discussed comparatively for selected materials: MoSe2, WSe2 and WS2. 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.

M. Koperski, M. R. Molas, A. Arora, K. Nogajewski, M. Bartos, J. Wyzula, D. Vaclavkova, P. Kossacki, M. Potemski
2D Materials 6, 015001 (2019)

Raman scattering from the bulk inactive out-of-plane B1_2g mode in few-layer MoTe2

Raman scattering from the out-of-plane vibrational modes (A1g/A'1), which originate from the bulk-inactive out-of-plane B1_2g mode, are studied in few-layer MoTe2. 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 MoTe2 is proposed to be responsible for the observed effects.

M. Grzeszczyk, K. Gołasa, M. R. Molas, K. Nogajewski, M. Zinkiewicz, M. Potemski, A. Wysmołek, A. Babiński
Sci. Rep. 8, 17745 (2018)

Singlet and triplet trions in WS2 monolayer encapsulated in hexagonal boron nitride

Embedding a WS2 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 WS2 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.

D. Vaclavkova, J. Wyzula, K. Nogajewski, M. Bartos, A. O. Slobodeniuk, C. Faugeras, M. Potemski, M. R. Molas
Nanotechnology 29, 325705 (2018)


Impact of environment on dynamics of exciton complexes in a WS2 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 WS2, 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.

T. Jakubczyk, K. Nogajewski, M. R. Molas, M. Bartos, W. Langbein, M. Potemski, J. Kasprzak
2D Materials 5, 031007 (2018)

Single Photon Emitters in Boron Nitride: More Than a Supplementary Material

We present comprehensive optical studies of recently discovered single photon sources in boron nitride, which appear in form of narrow lines emitting centres. Here, we aim to compactly characterise their basic optical properties, including the demonstration of several novel findings, in order to inspire discussion about their origin and utility. Initial inspection reveals the presence of narrow emission lines in boron nitride powder and exfoliated flakes of hexagonal boron nitride deposited on Si/SiO2 substrates. Generally rather stable, the boron nitride emitters constitute a good quality visible light source. However, as briefly discussed, certain specimens reveal a peculiar type of blinking effects, which are likely related to existence of meta-stable electronic states. More advanced characterisation of representative stable emitting centres uncovers a strong dependence of the emission intensity on the energy and polarisation of excitation. On this basis, we speculate that rather strict excitation selectivity is an important factor determining the character of the emission spectra, which allows the observation of single and well-isolated emitters. Finally, we investigate the properties of the emitting centres in varying external conditions. Quite surprisingly, it is found that the application of a magnetic field introduces no change in the emission spectra of boron nitride emitters. Further analysis of the impact of temperature on the emission spectra and the features seen in second-order correlation functions is used to provide an assessment of the potential functionality of boron nitride emitters as single photon sources capable of room temperature operation.


Optical response of monolayer, few-layer and bulk tungsten disulfide

We present a comprehensive optical study of thin films of tungsten disulfide (WS2) with layer thicknesses ranging from mono- to octalayer and in the bulk limit. It is shown that the optical band-gap absorption of monolayer WS2 is governed by competing resonances arising from one neutral and two distinct negatively charged excitons whose contributions to the overall absorption of light vary as a function of temperature and carrier concentration. The photoluminescence response of monolayer WS2is found to be largely dominated by disorder/impurity- and/or phonon-assisted recombination processes. The indirect band-gap luminescence in multilayer WS2 turns out to be a phonon-mediated process whose energy evolution with the number of layers surprisingly follows a simple model of a two-dimensional confinement. The energy position of the direct band-gap response (A and B resonances) is only weakly dependent on the layer thickness, which underlines an approximate compensation of the effect of the reduction of the exciton binding energy by the shrinkage of the apparent band gap. The A-exciton absorption-type spectra in multilayer WS2 display a non-trivial fine structure which results from the specific hybridization of the electronic states in the vicinity of the K-point of the Brillouin zone. The effects of temperature on the absorption-like and photoluminescence spectra of various WS2 layers are also quantified.

M. R. Molas, K. Nogajewski, A. O. Slobodeniuk, J. Binder, M. Bartos, M. Potemski
Nanoscale, 9, 13128-13141 (2017)


Optical properties of atomically thin transition metal dichalcogenides: observations and puzzles

Recent results on the optical properties of monolayer and few layers of semiconducting transition metal dichalcogenides are reviewed. Experimental observations are presented and discussed in the frame of existing models, highlighting the limits of our understanding in this emerging field of research. We first introduce the representative band structure of these systems and their interband optical transitions. The effect of an external magnetic field is then considered to discuss Zeeman spectroscopy and optical pumping experiments, both revealing phenomena related to the valley degree of freedom. Finally, we discuss the observation of single photon emitters in different types of layered materials, including wide band gap hexagonal boron nitride. While going through these topics, we try to focus on open questions and on experimental observations, which do not yet have a clear explanation.

M. Koperski, M. R. Molas, A. Arora, K. Nogajewski, A. O. Slobodeniuk, C. Faugeras and M. Potemski
Nanophotonics 6, 1289 (2017)


Raman scattering excitation spectroscopy in monolayer WS2

Resonant Raman scattering is investigated in monolayer WS2 at low temperature with the aid of an unconventional spectroscopy technique, i.e., Raman scattering excitation (RSE). The RSE spectrum is made up by sweeping the excitation energy, when the detection energy is fixed in resonance with excitonic transitions related to neutral and/or charged excitons. We demonstrate that the shape of the RSE spectrum strongly depends on a selected detection energy. The out-going resonance with the neutral exciton leads to an extremely rich RSE spectrum displaying several Raman scattering features not reported so far, while no clear effect on the associated background photoluminescence is observed. Instead, a strong enhancement of the emission due to the negatively charged exciton is apparent when the out-going photons resonate with this exciton. Presented results show that the RSE spectroscopy can be a useful technique to study electron-phonon interactions in thin layers of transition metal dichalcogenides.


Magnetic field induced polarization enhancement in monolayers of tungsten dichalcogenides: Effects of temperature

Optical orientation of localized/bound excitons is shown to be effectively enhanced by the application of magnetic fields as low as 20 mT in monolayer WS2. At low temperatures, the evolution of the polarization degree of different emission lines of monolayer WS2 with increasing magnetic fields is analyzed and compared to similar results obtained on a WSe2 monolayer. We study the temperature dependence of this effect up to T=60 K for both materials, focusing on the dynamics of the valley pseudospin relaxation. A rate equation model is used to analyze our data and from the analysis of the width of the polarization deep in magnetic field we conclude that the competition between the dark exciton pseudospin relaxation and the decay of the dark exciton population into the localized states are rather different in these two materials which are representative of the two extreme cases for the ratio of relaxation rate and depolarization rate.

T. Smoleński, T. Kazimierczuk, M. Goryca, M. R. Molas, K. Nogajewski, C. Faugeras, M. Potemski, P. Kossacki
2D Materials 5, 015023 (2017)

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