Giant Faraday rotation in atomically thin semiconductors.

Faraday rotation is a fundamental effect in the magneto-optical response of solids, liquids and gases. Materials with a large Verdet constant find applications in optical modulators, sensors and non-reciprocal devices, such as optical isolators. Here, we demonstrate that the plane of polarization of light exhibits a giant Faraday rotation of several degrees around the A exciton transition in hBN-encapsulated monolayers of WSe2 and MoSe2 under moderate magnetic fields. This results in the highest known Verdet constant of -1.9 × 107 deg T-1 cm-1 for any material in the visible regime. Additionally, interlayer excitons in hBN-encapsulated bilayer MoS2 exhibit a large Verdet constant (VIL ≈ +2 × 105 deg T-1 cm-2) of opposite sign compared to A excitons in monolayers. The giant Faraday rotation is due to the giant oscillator strength and high g-factor of the excitons in atomically thin semiconducting transition metal dichalcogenides. We deduce the complete in-plane complex dielectric tensor of hBN-encapsulated WSe2 and MoSe2 monolayers, which is vital for the prediction of Kerr, Faraday and magneto-circular dichroism spectra of 2D heterostructures. Our results pose a crucial advance in the potential usage of two-dimensional materials in ultrathin optical polarization devices.

Engineering 2D Material Exciton Line Shape with Graphene/h-BN Encapsulation.

Control over the optical properties of atomically thin two-dimensional (2D) layers, including those of transition metal dichalcogenides (TMDs), is needed for future optoelectronic applications. Here, the near-field coupling between TMDs and graphene/graphite is used to engineer the exciton line shape and charge state. Fano-like asymmetric spectral features are produced in WS2, MoSe2, and WSe2 van der Waals heterostructures combined with graphene, graphite, or jointly with hexagonal boron nitride (h-BN) as supporting or encapsulating layers. Furthermore, trion emission is suppressed in h-BN encapsulated WSe2/graphene with a neutral exciton red shift (44 meV) and binding energy reduction (30 meV). The response of these systems to electron beam and light probes is well-described in terms of 2D optical conductivities of the involved materials. Beyond fundamental insights into the interaction of TMD excitons with structured environments, this study opens an unexplored avenue toward shaping the spectral profile of narrow optical modes for application in nanophotonic devices.

Pressure Dependence of Intra- and Interlayer Excitons in 2H-MoS2 Bilayers.

The optical and electronic properties of multilayer transition metal dichalcogenides differ significantly from their monolayer counterparts due to interlayer interactions. The separation of individual layers can be tuned in a controlled way by applying pressure. Here, we use a diamond anvil cell to compress bilayers of 2H-MoS2 in the gigapascal range. By measuring optical transmission spectra, we find that increasing pressure leads to a decrease in the energy splitting between the A and the interlayer exciton. Comparing our experimental findings with ab initio calculations, we conclude that the observed changes are not due to the commonly assumed hydrostatic compression. This effect is attributed to the MoS2 bilayer adhering to the diamond, which reduces the in-plane compression. Moreover, we demonstrate that the distinct real-space distributions and resulting contributions from the valence band account for the different pressure dependencies of the inter- and intralayer excitons in compressed MoS2 bilayers.

Low-Divergence hBN Single-Photon Source with a 3D-Printed Low-Fluorescence Elliptical Polymer Microlens.

Efficiently collecting light from single-photon emitters is crucial for photonic quantum technologies. Here, we develop and use an ultralow fluorescence photopolymer to three-dimensionally print micrometer-sized elliptical lenses on individual precharacterized single-photon emitters in hexagonal boron nitride (hBN) nanocrystals, operating in the visible regime. The elliptical lens design beams the light highly efficiently into the far field, rendering bulky objective lenses obsolete. Using back focal plane imaging, we confirm that the emission is collimated to a narrow low-divergence beam with a half width at half-maximum of 2.2°. Using photon correlation measurements, we demonstrate that the single-photon character remains undisturbed by the polymer lens. The strongly directed emission and increased collection efficiency is highly beneficial for quantum optical experiments. Furthermore, our approach paves the way for a highly parallel fiber-based detection of single photons from hBN nanocrystals.

High-Performance Broadband Faraday Rotation Spectroscopy of 2D Materials and Thin Magnetic Films.

A Faraday rotation spectroscopy (FRS) technique is presented for measurements on the micrometer scale. Spectral acquisition speeds of about two orders of magnitude faster than state-of-the-art modulation spectroscopy setups are demonstrated. The experimental method is based on charge-coupled-device detection, avoiding speed-limiting components, such as polarization modulators with lock-in amplifiers. At the same time, FRS spectra are obtained with a sensitivity of 20 µrad ( 0.001 ° \[0.001{\bm{^\circ }}\] ) over a broad spectral range (525-800 nm), which is on par with state-of-the-art polarization-modulation techniques. The new measurement and analysis technique also automatically cancels unwanted Faraday rotation backgrounds. Using the setup, Faraday rotation spectroscopy of excitons is performed in a hexagonal boron nitride-encapsulated atomically thin semiconductor WS2 under magnetic fields of up to 1.4 T at room temperature and liquid helium temperature. An exciton g-factor of -4.4 ± 0.3 is determined at room temperature, and -4.2 ± 0.2 at liquid helium temperature. In addition, FRS and hysteresis loop measurements are performed on a 20 nm thick film of an amorphous magnetic Tb20 Fe80 alloy.

Quantitative Strain and Topography Mapping of 2D Materials Using Nanobeam Electron Diffraction.

It is known that 2D materials can exhibit a nonflat topography, which gives rise to an inherent strain. Since local curvature and strain influence mechanical, optical, and electrical properties, but are often difficult to distinguish from each other, a robust measurement technique is needed. In this study, a novel method is introduced, which is capable of obtaining quantitative strain and topography information of 2D materials with nanometer resolution. Relying on scanning nanobeam electron diffraction (NBED), it is possible to disentangle strain from the local sample slope. Using the positions of the diffraction spots of a specimen at two different tilts to reconstruct the locations and orientations of the reciprocal lattice rods, the local strain and slope can be simultaneously retrieved. We demonstrate the differences to strain measurements from a single NBED map in theory, simulation, and experiment. MoS2 monolayers with different shapes are used as simulation test structures. The slope and height information are recovered, as well as tensile and angular strain which have an absolute difference of less than 0.2% and 0.2° from the theoretical values. An experimental proof of concept using a freely suspended WSe2 monolayer together with a discussion of the accuracy of the method is provided.

Dark exciton anti-funneling in atomically thin semiconductors.

Transport of charge carriers is at the heart of current nanoelectronics. In conventional materials, electronic transport can be controlled by applying electric fields. Atomically thin semiconductors, however, are governed by excitons, which are neutral electron-hole pairs and as such cannot be controlled by electrical fields. Recently, strain engineering has been introduced to manipulate exciton propagation. Strain-induced energy gradients give rise to exciton funneling up to a micrometer range. Here, we combine spatiotemporal photoluminescence measurements with microscopic theory to track the way of excitons in time, space and energy. We find that excitons surprisingly move away from high-strain regions. This anti-funneling behavior can be ascribed to dark excitons which possess an opposite strain-induced energy variation compared to bright excitons. Our findings open new possibilities to control transport in exciton-dominated materials. Overall, our work represents a major advance in understanding exciton transport that is crucial for technological applications of atomically thin materials.

Covalent photofunctionalization and electronic repair of 2H-MoS2via nitrogen incorporation.

A route towards covalent functionalization of chemically inert 2H-MoS2 exploiting sulfur vacancies is explored by means of (TD)DFT and GW/BSE calculations. Functionalization via nitrogen incorporation at sulfur vacancies is shown to result in more stable covalent binding than via thiol incorporation. In this way, defective monolayer MoS2 is repaired and the quasiparticle band structure as well as the remarkable optical properties of pristine MoS2 are restored. Hence, defect-free functionalization with various molecules is possible. Our results for covalently attached azobenzene, as a prominent photo-switch, pave the way to create photoresponsive two-dimensional (2D) materials.

Strain tuning of the Stokes shift in atomically thin semiconductors.

Atomically thin layers of transition metal dichalcogenides (TMDC) have exceptional optical properties, exhibiting a characteristic absorption and emission at excitonic resonances. Due to their extreme flexibility, strain can be used to alter the fundamental exciton energies and line widths of TMDCs. Here, we report on the Stokes shift, i.e. the energetic difference of light absorption and emission, of the A exciton in TMDC mono- and bilayers. We demonstrate that mechanical strain can be used to tune the Stokes shift. We perform optical transmission and photoluminescence (PL) experiments on mono- and bilayers and apply uniaxial tensile strain of up to 1.2% in MoSe2 and WS2 bilayers. An A exciton red shift of -38 meV/% and -70 meV/% is found in transmission in MoSe2 and WS2, while smaller values of -27 meV/% and -62 meV/% are measured in PL, respectively. Therefore, a reduction of the Stokes shift is observed under increasing tensile strain. At the same time, the A exciton PL line widths narrow significantly with -14 meV/% (MoSe2) and -21 meV/% (WS2), demonstrating a drastic change in the exciton-phonon interaction. By comparison with ab initio calculations, we can trace back the observed shifts of the excitons to changes in the electronic band structure of the materials. Variations of the relative energetic positions of the different excitons lead to a decrease of the exciton-phonon coupling. Furthermore, we identify the indirect exciton emission in bilayer WS2 as the ΓK transition by comparing the experimental and theoretical gauge factors.

Excited-State Trions in Monolayer WS_{2}.

We discover an excited bound three-particle state, the 2s trion, appearing energetically below the 2s exciton in monolayer WS_{2}, using absorption spectroscopy and ab initio GW and Bethe-Salpeter equation calculations. The measured binding energy of the 2s trion (22 meV) is smaller compared to the 1s intravalley and intervalley trions (37 and 31 meV). With increasing temperature, the 1s and 2s trions transfer their oscillator strengths to the respective neutral excitons, establishing an optical fingerprint of trion-exciton resonance pairs. Our discovery underlines the importance of trions for the entire excitation spectrum of two-dimensional semiconductors.

Space- and time-resolved UV-to-NIR surface spectroscopy and 2D nanoscopy at 1 MHz repetition rate.

We describe a setup for time-resolved photoemission electron microscopy with aberration correction enabling 3 nm spatial resolution and sub-20 fs temporal resolution. The latter is realized by our development of a widely tunable (215-970 nm) noncollinear optical parametric amplifier (NOPA) at 1 MHz repetition rate. We discuss several exemplary applications. Efficient photoemission from plasmonic Au nanoresonators is investigated with phase-coherent pulse pairs from an actively stabilized interferometer. More complex excitation fields are created with a liquid-crystal-based pulse shaper enabling amplitude and phase shaping of NOPA pulses with spectral components from 600 to 800 nm. With this system we demonstrate spectroscopy within a single plasmonic nanoslit resonator by spectral amplitude shaping and investigate the local field dynamics with coherent two-dimensional (2D) spectroscopy at the nanometer length scale ("2D nanoscopy"). We show that the local response varies across a distance as small as 33 nm in our sample. Further, we report two-color pump-probe experiments using two independent NOPA beamlines. We extract local variations of the excited-state dynamics of a monolayered 2D material (WSe2) that we correlate with low-energy electron microscopy (LEEM) and reflectivity measurements. Finally, we demonstrate the in situ sample preparation capabilities for organic thin films and their characterization via spatially resolved electron diffraction and dark-field LEEM.

Supercontinuum second harmonic generation spectroscopy of atomically thin semiconductors.

Two-dimensional semiconductors have recently emerged as promising materials for novel optoelectronic devices. In particular, they exhibit favorable nonlinear optical properties. Potential applications include broadband and ultrafast light sources, optical signal processing, and generation of nonclassical light states. The prototypical nonlinear process second harmonic generation (SHG) is a powerful tool to gain insight into nanoscale materials because of its dependence on crystal symmetry. Material resonances also play an important role in the nonlinear response. Notably, excitonic resonances critically determine the magnitude and spectral dependence of the nonlinear susceptibility. We perform ultrabroadband SHG spectroscopy of atomically thin semiconductors by using few-cycle femtosecond infrared laser pulses. The spectrum of the second harmonic depends on the investigated material, MoS2 or WS2, and also on the spectral and temporal shape of the fundamental laser pulses used for excitation. Here, we present a method to remove the influence of the laser by normalization with the flat SHG response of thin hexagonal boron nitride crystals. Moreover, we exploit the distinct angle dependence of the second harmonic signal to suppress two-photon photoluminescence from the semiconductor monolayers. Our experimental technique provides the calibrated frequency-dependent nonlinear susceptibility χ(2)(ω) of atomically thin materials. It allows for the identification of the prominent A and B exciton resonances, as well as excited exciton states.

Interlayer excitons in bilayer MoS2 under uniaxial tensile strain.

Atomically thin semiconducting transition metal dichalcogenides (TMDCs) have unique mechanical and optical properties. They are extremely flexible and exhibit a strong optical absorption at their excitonic resonances. Excitons in TMDC monolayers are strongly influenced by mechanical strain. Their energy shifts and even their line widths change. In bilayers, intralayer excitons with electrons and holes residing in the same layer also shift their energy with the applied strain. Recently, interlayer excitons with electrons and holes in different layers have been observed in bilayer MoS2 at room temperature. Here, we report on the behavior of interlayer excitons in bilayer MoS2 under uniaxial tensile strain of up to 1.6%. By recording the differential transmission spectra for different strain values, we derive a gauge factor of -47 meV per % for the energy shift of the interlayer exciton, which is similar to -49 meV per % for the intralayer A and B excitons. Our finding confirms the origin of the interlayer exciton at the K point in the Brillouin zone, with the electron located in one layer and the hole delocalized over two layers. Furthermore, our work paves the way for future straintronic devices based on interlayer excitons.

Electroluminescence from multi-particle exciton complexes in transition metal dichalcogenide semiconductors.

Light emission from higher-order correlated excitonic states has been recently reported in hBN-encapsulated monolayer WSe2 and WS2 upon optical excitation. These exciton complexes are found to be bound states of excitons residing in opposite valleys in momentum space, a promising feature that could be employed in valleytronics or other novel optoelectronic devices. However, electrically-driven light emission from such exciton species is still lacking. Here we report electroluminescence from bright and dark excitons, negatively charged trions and neutral and negatively charged biexcitons, generated by a pulsed gate voltage, in hexagonal boron nitride encapsulated monolayer WSe2 and WS2 with graphene as electrode. By tailoring the pulse parameters we are able to tune the emission intensity of the different exciton species in both materials. We find the electroluminescence from charged biexcitons and dark excitons to be as narrow as 2.8 meV.

Magnetic and Optical Properties of Gold-Coated Iron Oxide Nanoparticles.

In this work, magnetic and optical properties of magnetic nanoparticles were investigated, where the particles of iron oxide were prepared with a co-precipitation route and the component of gold was built up by reduction of AuCl4- on the surface of iron oxide to assemble nanocomposite structures in the form of an electrostatic stabilized suspension. The size of the particles obtained with TEM increased from of 8.9 ± 2.7 to 16 ± 6 nm after the procedure of hybridisation. In order to distinguish the impact of the gold on the optical properties, UV-Vis and Raman spectroscopy techniques were used. Magnetic properties were studied in the temperature range of 5-300 K and the superparamagnetic state of MNPs at room temperature was confirmed for both systems.

Revisiting the Buckling Metrology Method to Determine the Young's Modulus of 2D Materials.

Measuring the mechanical properties of 2D materials is a formidable task. While regular electrical and optical probing techniques are suitable even for atomically thin materials, conventional mechanical tests cannot be directly applied. Therefore, new mechanical testing techniques need to be developed. Up to now, the most widespread approaches require micro-fabrication to create freely suspended membranes, rendering their implementation complex and costly. Here, a simple yet powerful technique is revisited to measure the mechanical properties of thin films. The buckling metrology method, that does not require the fabrication of freely suspended structures, is used to determine the Young's modulus of several transition metal dichalcogenides (MoS2 , MoSe2 , WS2 , and WSe2 ) with thicknesses ranging from 2 to 10 layers. The obtained values for the Young's modulus and their uncertainty are critically compared with previously published results, finding that this simple technique provides results which are in good agreement with those reported using other highly sophisticated testing methods. By comparing the cost, complexity, and time required for the different methods reported in the literature, the buckling metrology method presents certain advantages that make it an interesting mechanical test tool for 2D materials.

Thickness-Dependent Differential Reflectance Spectra of Monolayer and Few-Layer MoS2, MoSe2, WS2 and WSe2.

The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques opened the door to study the optical properties of these nanomaterials. We presented a comprehensive study of the differential reflectance spectra of 2D semiconducting transition metal dichalcogenides (TMDCs), MoS2, MoSe2, WS2, and WSe2, with thickness ranging from one layer up to six layers. We analyzed the thickness-dependent energy of the different excitonic features, indicating the change in the band structure of the different TMDC materials with the number of layers. Our work provided a route to employ differential reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2, and WSe2.

Facile synthesis of WS2 nanotubes by sulfurization of tungsten thin films: formation mechanism, and structural and optical properties.

While 2D layers of WS2 have been extensively studied, there are very few investigations of WS2 nanotubes. These have usually been grown via a 2-step process involving a WO3-x intermediate. We report a simple process for the synthesis of WS2 nanotubes via the sulfurization of tungsten films under appropriate conditions and present details of their structural and optical properties that help elucidate the formation mechanism. Electron-beam evaporated films of tungsten are sulfurized under flowing N2 gas at 950-1000 °C temperature under atmospheric pressure to obtain WS2 nanotubes. High-resolution scanning and transmission electron microscopy studies show that 2D WS2 flakes curl up and wrap around themselves to form nanotubes. Interlayer spacings in both 'a' and 'c' directions are slightly smaller than the corresponding values in bulk and thin film WS2. Micro-photoluminescence and micro-transmission studies show a resonance that seems intrinsic to the WS2 nanotubes since it cannot be related to the known optical characteristics of WS2 flakes.

Valley-contrasting optics of interlayer excitons in Mo- and W-based bulk transition metal dichalcogenides.

Recently, spatially indirect ("interlayer") excitons have been discovered in bulk 2H-MoTe2. They are theoretically predicted to exist in other Mo-based transition metal dichalcogenides (TMDCs) and are expected to be present in W-based TMDCs as well. We investigate interlayer excitons (XIL) in bulk 2H-MoSe2 and 2H-WSe2 using valley-resolved magneto-reflectance spectroscopy under high magnetic fields of up to 29 T combined with ab initio GW-BSE calculations. In the experiments, we observe interlayer excitons in MoSe2, while their signature is surprisingly absent in WSe2. In the calculations, we find that interlayer excitons exist in both Mo- and W-based TMDCs. However, their energetic positions and their oscillator strengths are remarkably different. In Mo-based compounds, the interlayer exciton resonance XIL is clearly separated from the intralayer exciton X1sA and has a high amplitude. In contrast, in W-based compounds, XIL is close in energy to the intralayer A exciton X1sA and possesses a small oscillator strength, which explains its absence in the experimental data of WSe2. Our combined experimental and theoretical observations demonstrate that interlayer excitons can gain substantial oscillator strength by mixing with intralayer states and hence pave the way for exploring interlayer exciton physics in Mo-based bulk transition metal dichalcogenides.

Incorporation of oxygen atoms as a mechanism for photoluminescence enhancement of chemically treated MoS2.

Chemical treatments to enhance photoluminescence (PL) in MoS2 have been explored extensively by experimental means in recent years. However, satisfactory theoretical explanations of the underlying mechanisms remain elusive. In this work, the surface reactions of the superacid bis(trifluoromethane)-sulfonimide (TFSI), hydrogen peroxide (H2O2), molecular oxygen (O2), and sulfuric acid (H2SO4) on a defective MoS2 monolayer have been studied using first principles calculations. An oxygen transfer reaction into a sulfur vacancy with a low activation barrier and thus significant reaction rates already at room temperature has been found. Band structure unfolding techniques show that the incorporation of oxygen atoms into sulfur vacancies restores the band structure of pristine MoS2, which is predicted to have a high PL quantum yield. PL spectroscopy is used to examine the effect of chemical treatment on PL intensity. Our experimental findings support our theoretical predictions, as PL in MoS2 is enhanced by up to a factor 20 after treatment with H2O2 or H2SO4, while the spectral shape is only slightly altered.