Substrate influence on transition metal dichalcogenide monolayer exciton absorption linewidth broadening
Fuhui Shao, Steffi Y. Woo, Nianjheng Wu, Robert Schneider, Andrew J. Mayne, Steffen Michaelis de Vasconcellos, Ashish Arora, Benjamin J. Carey, Johann A. Preuß,… 展开作者
Fuhui Shao, Steffi Y. Woo, Nianjheng Wu, Robert Schneider, Andrew J. Mayne, Steffen Michaelis de Vasconcellos, Ashish Arora, Benjamin J. Carey, Johann A. Preuß, Noémie Bonnet, Cecilia Mattevi, Kenji Watanabe, Takashi Taniguchi, Zhichuan Niu, Rudolf Bratschitsch, Luiz H. G. Tizei
The excitonic states of transition metal dichacolgenide (TMD) monolayers are heavily influenced by their external dielectric environment based on the substrate used. In this work, various wide bandgap dielectric materials, namely hexagonal boron nitride (\textit{h}-BN) and amorphous silicon nitride (Si$_3$N$_4$), under different configurations as support or encapsulation material for WS$_2$ monolayers are investigated to disentangle the factors contributing to inhomogeneous broadening of exciton absorption lines in TMDs using electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). In addition, monolayer roughness in each configuration was determined from tilt series of electron diffraction patterns by assessing the broadening of diffraction spots by comparison with simulations. From our experiments, the main factors that play a role in linewidth broadening can be classified in increasing order of importance by: monolayer roughness, surface cleanliness, and substrate-induced charge trapping. Furthermore, because high-energy electrons are used as a probe, electron beam-induced damage on bare TMD monolayer is also revealed to be responsible for irreversible linewidth increases. \textit{h}-BN not only provides clean surfaces of TMD monolayer, and minimal charge disorder, but can also protect the TMD from irradiation damage. This work provides a better understanding of the mechanisms by which \textit{h}-BN remains, to date, the most compatible material for 2D material encapsulation, facilitating the realization of intrinsic material properties to their full potential.
We obtain an analytical expression for the linewidth of the 1s-exciton as a function of temperature in transition metal dichalcogenides. The total linewidth, as function of temperature, is dominated by three contributions: (i) the radiative decay (essentially temperature independent); (ii) the phonon-induced intravalley scattering; (iii) the phonon-induced intervalley scattering. Our approach uses a variational \emph{Ansatz} to solve the Wannier equation allowing for an analytical treatment of the excitonic problem, including rates of the decay dynamics. Our results are in good agreement with …
Monolayer transition metal dichalcogenides (TMDs) are known to be highly sensitive to externally applied tensile or compressive strain. In particular, strain can be exploited as a tool to control the optical response of TMDs. However, the role of excitonic effects under strain has not been fully understood yet. Utilizing the strain-induced modification of electron and phonon dispersion obtained by first principle calculations, we present in this work microscopic insights into the strain-dependent optical response of various TMD materials. In particular, we explain recent experiments on the cha…
We report on linewidth analysis of optical transitions in InAs/GaAs coupled quantum dots as a function of bias voltage, temperature, and tunnel coupling strength. A significant line broadening up to 100 $μ$eV is observed at hole tunneling resonances where the coherent tunnel coupling between spatially direct and indirect exciton states is maximized, corresponding to a phonon-assisted transition rate of 150 ns${}^{-1}$ at 20 K. With increasing temperature, the linewidth shows broadening characteristic of single-phonon transitions. The linewidth as a function of tunnel coupling strength tracks t…
Strong Coulomb interactions in atomically thin semiconductors like monolayer WSe$_2$ induce not only tightly bound excitons, but also make their optical properties very sensible to doping. By utilizing a microscopic theory based on the excitonic Heisenberg equations of motion, we systematically determine the influence of doping on the excitonic linewidth, lineshift, and oscillator strength. We calculate trion resonances and demonstrate that the Coulomb coupling of excitons to the trionic continuum generates a non-Markovian interference, which, due to a time retardation, builds up a phase respo…
Monolayer Transition Metal Dichalcogenides (TMDS) are highly luminescent materials despite being sub-nanometer thick due to the ultra-short ($<1$ ps) radiative lifetime of the strongly bound bright excitons hosted by these materials. The intrinsically short radiative lifetime results in a large broadening in the exciton band with a magnitude that is about two orders greater than the spread of the light cone itself. The situation calls for a need to revisit the conventional light cone picture. We present a modified light cone concept which places the light line $(\hbar cQ)$ as the generalized l…
Embedding a monolayer of a transition metal dichalcogenide in a high-Q optical cavity results in the formation of distinct exciton polariton modes. The polaritons are affected by the strong exciton-phonon interaction in the monolayer. We use a time convolutionless master equation to calculate the phonon influence on the spectra of the polaritons. We discuss the non-trivial dependence of the line shapes of both branches on temperature and detuning. The peculiar polariton dispersion relation results in a linewidth of the lower polariton being largely independent of the coupling to acoustic phono…
Strong reduction of exciton-phonon coupling in high crystalline quality single-wall carbon nanotubes: a new insight into broadening mechanisms and exciton localization
The recent accessibility of high quality, charge neutral monolayer transition metal dichalcogenides with narrow exciton linewidths at the homogeneous limit provides an ideal platform to study excitonic many-body interactions. In particular, the possibility to manipulate coherent exciton-exciton interactions, which govern the ultrafast nonlinear optical response, by applying an external magnetic field has not been considered so far. We address this discrepancy by presenting a nonlinear microscopic theory in the coherent limit for optical excitations in the presence of out-of-plane, in-plane, an…
Spectral analysis of electron spin resonance (ESR) is a powerful technique for various investigations including characterization of spin systems, measurements of spin concentration, and probing spin dynamics. The nitrogen-vacancy (NV) center in diamond is a promising magnetic sensor enabling improvement of ESR sensitivity to the level of a single spin. Therefore, understanding the nature of NV-detected ESR (NV-ESR) spectrum is critical for applications to nanoscale ESR. Within this work we investigate the linewidth of NV-ESR from single substitutional nitrogen centers (called P1 centers). NV-E…
Localized excitons play a vital role in the optical response of monolayers of transition metal dichalcogenides and can be exploited as single photon sources for quantum information technology. While the optical properties of such localized excitons are vastly studied, the ultrafast capture process of delocalized excitons into localized potentials is largely unexplored. We perform quantum kinetic calculations of exciton capture via acoustic and optical phonons showing that efficient capture takes place on an ultrafast time scale. The polaron formation in the low-temperature limit leads to highe…
Recent experimental studies of out-of-plane straining geometries of transition metal dichalchogenide (TMD) monolayers have demonstrated sufficient band gap renormalisation for device application such as single photon emitters. Here, a simple continuum-mechanical plate-theory approach is used to estimate the topography of TMD monolayers layered atop nanopillar arrays. From such geometries, the induced conduction band potential and band gap renormalisation is given, demonstrating a curvature of the potential that is independent of the height of the deforming nanopillar. Additionally, with a semi…
Ensembles of bismuth donor spins in silicon are promising storage elements for microwave quantum memories due to their long coherence times which exceed seconds. Operating an efficient quantum memory requires achieving critical coupling between the spin ensemble and a suitable high-quality factor resonator -- this in turn requires a thorough understanding of the lineshapes for the relevant spin resonance transitions, particularly considering the influence of the resonator itself on line broadening. Here, we present pulsed electron spin resonance measurements of ensembles of bismuth donors in n…
Establishing a highly efficient photon-emitter interface where the intrinsic linewidth broadening is limited solely by spontaneous emission is a key step in quantum optics. It opens a pathway to coherent light-matter interaction for, e.g., the generation of highly indistinguishable photons, few-photon optical nonlinearities, and photon-emitter quantum gates. However, residual broadening mechanisms are ubiquitous and need to be combated. For solid-state emitters charge and nuclear spin noise is of importance and the influence of photonic nanostructures on the broadening has not been clarified. …
Enhanced Coulomb interactions in monolayer transition metal dichalcogenides cause tightly bound electron-hole pairs (excitons) which dominate their linear and nonlinear optical response. The latter includes bleaching, energy renormalizations, and higher-order Coulomb correlation effects like biexcitons and excitation-induced dephasing (EID). While the first three are extensively studied, no theoretical footing for EID in exciton dominated semiconductors is available so far. In this study, we present microscopic calculations based on excitonic Heisenberg equations of motion and identify the cou…
Disorder derived from defects or strain in monolayer TMDs can lead to a dramatic change in the physical behavior of the interband excitations, producing inhomogeneous spectral broadening and localization; leading to radiative lifetime increase. In this study, we have modeled the disorder in the surface of the sample through a randomized potential in monolayer WSe2. We show that this model allows us to simulate the spectra of localized exciton states as well as their radiative lifetime. In this context, we give an in depth study of the influence of the disorder potential parameters on the optic…
Excitonic quasi-particles, excitons/trions/bi-excitons, and their coupling with phonons and charge carriers play a crucial role in controlling the optical properties of atomically thin semiconducting 2D materials. In this work, we unravelled the dynamics of excitons/trions and their coupling with phonons and charge carriers in a few layers vertically and horizontally aligned MoS2. We observed trion signature up to the highest recorded temperature (330 K) in both systems and have shown that the dynamics of excitons/trions and their coupling with phonons and electrons are more affected in vertic…
We analyze the dielectric-function spectra of low dimensional transition metal dichalcogenides (TMDCs) using a fully analytical model of the complex dielectric function that is applicable in fractional dimensional space. We extract the dimensionalities of the $A$ and $B$ excitons as well as their Lorentzian broadening widths by fitting the model to experimental data in the spectral range of photon energies (1.5 - 3 eV). Our results show the significant contribution of the lowest ground exciton state to the dielectric properties of exemplary monolayer materials (MoS$_2$, MoSe$_2$ and WSe$_2$). …
The reduced dielectric screening in atomically thin transition metal dichalcogenides allows to study the hydrogen-like series of higher exciton states in optical spectra even at room temperature. The width of excitonic peaks provides information about the radiative decay and phonon-assisted scattering channels limiting the lifetime of these quasi-particles. While linewidth studies so far have been limited to the exciton ground state, encapsulation with hBN has recently enabled quantitative measurements of the broadening of excited exciton resonances. Here, we present a joint experiment-theory …
Demonstrating Kondo behavior by temperature-dependent scanning tunneling spectroscopy
Department of Applied Physics, Aalto University（阿尔托大学应用物理系）；Departamento de Polímeros y Materiales Avanzados: Física, Química y Tecnología, Universidad del País Vasco UPV/EHU（巴斯克大学聚合物与先进材料系）；Department of Chemistry, University of Zurich（苏黎世大学化学系）；Department of Chemistry, Biochemistry and Pharmaceutical Sciences, University of Bern（伯尔尼大学化学、生物化学和药学系）
Layered transition metal dichalcogenides (TMDCs) host a variety of strongly bound exciton complexes that control the optical properties in these materials. Apart from spin and valley, layer index provides an additional degree of freedom in a few-layer thick film. Here we show that in a few-layer TMDC film, the wavefunctions of the conduction and valence band edge states contributing to the K (K') valley are spatially confined in the alternate layers - giving rise to direct (quasi-)intra-layer bright exciton and lower-energy inter-layer dark excitons. Depending on the spin and valley configurat…
